This portobello burger skips the bean patty entirely. Instead, portobello mushrooms and zucchini get blended, seasoned with a barbecue dry rub, and grilled into patties — topped with caramelized onions and fresh mashed avocado. The whole thing comes together in 20 minutes with one pan and a grill.
What makes this version work
Two things carry this recipe. First, blending the mushrooms and zucchini together rather than grilling whole caps gives you a patty that holds its shape and absorbs the dry rub all the way through — not just on the surface. Second, the caramelized onions are cooked in a mix of water and agave (or maple syrup) over low-medium heat, which pulls out their natural sweetness without burning them. That liquid base also means you don’t need a second pan with butter or extra oil. Get the onions started before anything else, because they take the longest and you can run the rest of the recipe while they cook down.
Mistakes to avoid
- Skipping the squeeze on the zucchini: Zucchini holds a lot of water. After slicing, press it briefly between paper towels before it goes into the food processor. Wet zucchini makes patties that fall apart on the grill.
- Over-processing the mix: Pulse the mushrooms and zucchini — don’t run the processor continuously. You want a coarse, moldable texture, not a smooth paste. A paste won’t hold a patty shape.
- Grilling on a cold grate: A properly preheated grill (high heat) sears the outside of the patty quickly and helps it release cleanly. A lukewarm grate causes sticking and broken patties.
- Mashing the avocado too early: Avocado browns fast once it’s exposed to air. Mash it during the last two minutes of grilling, right before you build the burgers — not at the start of cooking.
- Using a dry rub with a lot of added sugar: Some commercial BBQ rubs are heavy on sugar, which scorches quickly at high grill heat. Check the label; if sugar is the first or second ingredient, use less or switch to a more savory blend.
Grilled Portobello Burger with Caramelized Onions
Equipment
- Food processor
- Large mixing bowl
- Grill or grill pan
- 10-inch non-stick skillet or cast iron skillet
- Spatula (wide, flat)
- Measuring spoons
Ingredients
- 6 medium hamburger buns brioche or sesame seed
- 1 pound portobello mushrooms stems removed and roughly chopped, about 450g
- 1 teaspoon olive oil for seasoning
- 2 medium zucchinis sliced, about 14 oz / 400g
- ½ cup yellow onion sliced thinly, about 75g
- 3 tablespoons barbecue dry rub about 20g
- 1 tablespoon olive oil for cooking, 15ml
- 1 tablespoon agave syrup or maple syrup or simple syrup, 15ml
- 3 oz water for caramelizing onions, 90ml
- 1 medium avocado mashed, about 150g flesh
- ½ teaspoon salt or to taste
- ¼ teaspoon ground black pepper or to taste
Instructions
- Roughly chop the portobello mushrooms and slice the zucchini. Place them in a large mixing bowl with 1 teaspoon of olive oil and 3 tablespoons of barbecue dry rub. Toss until the vegetables are evenly coated. Transfer to a food processor and pulse 8-10 times until the mixture is finely chopped but not puréed — you want some texture, not a paste.1 pound portobello mushrooms, 1 teaspoon olive oil, 2 medium zucchinis, 3 tablespoons barbecue dry rub
- Divide the mixture into 6 equal portions (about 4 oz / 115g each). Roll each into a ball, then flatten into patties about ¾ inch (2cm) thick. Refrigerate on a parchment-lined plate for 10 minutes to help them firm up before grilling.
- While the patties chill, start the caramelized onions. Heat 1 tablespoon of olive oil in a skillet over low-medium heat. Add the sliced onions, 3 oz (90ml) water, 1 tablespoon agave syrup, and a pinch of salt and pepper. Cook for 15-20 minutes, stirring occasionally, until the onions are deeply golden and jammy. Reduce heat if they start to brown too quickly.1/2 cup yellow onion, 1 tablespoon olive oil, 1 tablespoon agave syrup, 3 oz water, 1/2 teaspoon salt, 1/4 teaspoon ground black pepper
- Preheat your grill to high heat (around 450°F / 230°C). Lightly oil the grill grates. Place the chilled patties on the grill and cook for 3-4 minutes per side until charred grill marks form and the patties are heated through. Only flip once — moving them repeatedly will break them apart.
- While the patties grill, place the hamburger buns cut-side down on the grill for 1-2 minutes until lightly toasted. Mash the avocado in a small bowl with a pinch of salt.6 medium hamburger buns, 1 medium avocado, 1/2 teaspoon salt
- To assemble, spread mashed avocado on the bottom bun, place the grilled patty on top, pile on the caramelized onions, and close with the toasted top bun. Serve immediately.6 medium hamburger buns, 1 medium avocado
Notes
Nutrition
Your questions, answered
Can I cook these patties on a stovetop instead of a grill?
Yes — a cast iron skillet over medium-high heat works well. Cook 3 minutes per side the same as the grill, and you can use the same pan you caramelized the onions in once they’re done, which saves on cleanup.
How do I keep the patties from falling apart when I flip them?
Make sure the patties are chilled before they hit the heat — 10 minutes in the fridge after shaping firms them up noticeably. Also, only flip once; moving them around repeatedly breaks them down before a crust can form.
What can I use instead of agave for the caramelized onions?
Maple syrup or simple syrup both work, as the recipe notes. Honey also works in the same quantity. Skip the egg in the mix — it makes the patty mushy — but for the onions, any liquid sweetener with a similar consistency to agave is a direct swap.
More burger recipes to try
- Veggie Sauce Burger Perfection
- Five Guys Veggie Sandwich Style
- Halloumi King Burger Homage
- Veggie Whopper Copycat Recipe