In a 2-quart roasting pan, put eggplant, garlic and onion quarters, roast at 450°F in the oven for approximately 10 to 15 minutes.
Turn off the oven and leave the eggplant to cool in the oven. I find this ensures everything is soft without overcooking it since you will be frying these burgers after. You can also make this part the day before just store in the fridge, and I recommend allowing it to warm up a bit before the next steps.
When cool enough to handle, dump it in a food processor or high-speed blender, and add baby spinach and mushrooms (washed and stem removed) to puree until smooth.
Transfer to a large mixing bowl. There may still be chunks of eggplant; mash them out with a potato masher, now you get to get your hands dirty (and one of the reasons why the mix shouldn’t be hot!).
Add the chickpea flour and ½ of the breadcrumbs (keep both products handy, you may need more). Mix well into the eggplant; keep adding breadcrumbs and chickpea flour until you get a consistency that forms a nice, not crumbly patty. Be careful because if you put too much, you cannot undo it! I like to use my hands a bit to get everything well moistened.
Add oil to the frying pan and make sure the pan is well coated. Heat oil over medium heat, then carefully form 4 patties flattened to desired thickness (I like them thick and juicy!) and fry for about 6 to 7 minutes per side (until golden). Use a large spatula for flipping without breaking!
Serve on its own, or you can put it into a sandwich.
Notes
These are when cooked. They just need to be thawed then baked or fried up again, be careful, though as the water content changes, so does the texture.