Combine all the ingredients for the marinade in a large bowl. Reserve 2 Tbs for the bell peppers. And set 1Tbs aside for seasoning the lettuce wraps after cooking.
Pour remainder of the marinade into a large zip-top bag, add the steak, and refrigerate while preparing the rest of the food.
Slice the the onions and bell peppers.
In a medium sized skillet, over medium high heat, heat 1 tablespoon olive oil and add the onions and allow to cook for 5 minutes, or until softened and fragrant.
Add the bell peppers and the reserved 2 Tbs of the marinade mixture.
Cook for about 3-5 minutes for tender but firm peppers, or 6-8 minutes for soft peppers. Then remove from pan and set aside.
In the skillet the peppers were cooked in, melt butter and sear the steak 3 minutes on each side.
Remove steak from heat, and slice into thin slices. If desired doneness is not reached, cook longer. Return strips of steak to the skillet, and add the reserved steak marinade to the skillet, cook until just heated, about 1 minute.
Assemble lettuce wraps by placing a spoonful of vegetable filling into the center of each leaf, add strips of steak then top with avocado, a squeeze of lime, and fresh cilantro. Or your favorite toppings.