Whisk the beer, brown sugar, salt, pepper, garlic powder, black pepper, cumin, and serrano smoked chilli powder together in a large bowl.
Add the beef to the mixture and lightly mix with your hands. Form the meat mixture into a ball. Cover and refrigerate for about 2 hours to incorporate the flavours.
Spray the grill with grilling spray and preheat to medium-high heat. When the grill is hot, grill the burgers until your desired doneness. Grill the burgers for 3 to 4 minutes on each side for medium-rare. If you overcook the marinated burgers, they will become tough.
Cook the caramelized onions by heating oil and butter on medium-low in an extra-large skillet. (You may also divide the onions into two pans, but then divide the butter and olive oil.) Add half the onions to the pan and cook until they are slightly cooked down.
Add the remainder of the onions, season with salt and pepper and continue cooking until all the onions have cooked down. (If you are doing two pans, this speeds up the process. When both pans of onions have cooked down slightly, add all the onions to one extra-large pan, season and continue with the recipe.) After the onions are somewhat cooked down, add the brown sugar, stir gently. Reduce the heat, then occasionally stir for about 50 minutes or until the onions have reduced, look golden, and are tender.
You can bring the onions to room temperature, place them in a covered container, and refrigerate them for up to three days.