Preheat your outdoor grill to medium-high heat.
Add all the ingredients listed under "burger", except for the avocado, to a fairly large mixing bowl and knead by hand until well combined. Form into 12 thin, equal patties, of approximately 90g (3oz) each.
Slice the avocados in half crosswise, remove the pit and then slice each half into 3 equal slices; keep the 2 larger slices to stuff the burgers and throw the smaller ends into a small bowl to make the spicy mayo.
To that bowl, add paleo mayo and seasonings listed under "Spicy Mayo" and mash roughly with a fork to combine.
To build the burgers, place one avocado slice on six of the patties, then drop a spoonful of the mayo right in the center of the avocado slices. Cover with a second meat patty, then carefully pick this up in your hands and pinch the edges with your fingers to make sure your patties are nicely sealed. Finish forming the patties with your hands so they become beautiful and nice and round.
Brush the patties with avocado oil, sprinkle with salt and pepper and grill over medium-high heat for about 4-6 minutes per side, depending on desired level of doneness.
Serve on a bed of lettuce with caramelized onions and sautéed mushrooms.