Start by slicing the bell peppers into thin strips and the onion into half-moons. Keep them ready beside your stove.
Heat a large skillet over medium-high heat and add the olive oil. When it shimmers and starts to crackle, add the sliced peppers and onions in an even layer.
Cook the vegetables, stirring occasionally, for about 8-10 minutes until they develop deep caramel color and become tender with slightly charred edges.
Add the minced garlic to the skillet and cook for about 30 seconds, stirring until fragrant and just golden.
Sprinkle the fajita seasoning over the vegetables and toss well to coat evenly. Continue cooking for another 2 minutes until aromatic.
Meanwhile, in a separate pan or after removing the vegetables, quickly sauté the sliced zucchini until just tender and lightly browned, about 3-4 minutes.
Combine the cooked zucchini with the caramelized peppers and onions, then squeeze the fresh lime juice over everything for brightness. Toss gently to combine.
Remove the skillet from heat once the vegetables are tender, caramelized, and fragrant with smoky spices. Let them rest for a minute to settle the flavors.
Serve the caramelized vegetable mixture warm in tortillas, garnished with fresh cilantro or avocado slices if desired. Enjoy the smoky, sweet, and tender fajitas immediately.