Place the fish on a plate and sprinkle with the cumin, salt, pepper and chili powder, if you want a kick. Drizzle with olive oil and the juice of 1 lime.
Preheat a skillet over medium- high heat. Add the fish and cook until golden on the outside. Remove from the skillet and transfer to a plate.
Combine the avocado, juice of 1 lime, onion, cilantro, salt and pepper and set aside.
Heat the tortillas in an ungreased nonstick pan and wrap the tortillas in a clean towel to keep them warm.
To assemble the tacos, place an equal amount of the tilapia in each tortilla, along with a big spoonful of the avocado mixture and top with shredded lettuce. Serve with your favorite salsa on the side.