Heat olive oil in a skillet. Sauté onion until translucent, then add garlic. Mix well.
Place tilapia on the skillet and cook until the flesh starts to flake. Add jalapeño peppers, tomatoes, cilantro and lime juice. Sauté over medium-high heat for about 5 minutes, breaking up the fish with the spoon to get everything mixed well; season to taste with salt and pepper.
Meanwhile, heat tortillas on a skillet a few minutes on each side to warm (no oil needed). Serve a little over 1/4 cup of fish on each warmed tortillas with a slice or 2 of avocado and enjoy!