Preheat the oven to 450°F (232°C). Lightly spray a large rimmed baking sheet with non-stick cooking spray.
Mix the fajita seasoning: In a small bowl, stir together 2 tsp chili powder, 1 tsp pepper, and ½ tsp of salt, onion powder, cumin, cayenne, and paprika.
Prepare the vegetables: In a large mixing bowl, combine the sliced bell peppers and red onion. Drizzle with olive oil and toss to coat evenly.
Season the veggies: Sprinkle the seasoning mix and minced garlic over the peppers and onions. Toss well to coat.
Add the shrimp: Add the raw shrimp to the same bowl and gently toss everything together until the shrimp are lightly coated with the seasoning.
Spread onto the baking sheet: Arrange the shrimp and vegetables in a single, even layer on the prepared sheet pan.
Roast: Bake for 6–8 minutes, or until the shrimp are just starting to turn pink and the veggies begin to soften.
Broil: Gently toss the shrimp and vegetables, then switch the oven to broil. Cook for an additional 2–3 minutes, or until the shrimp are fully cooked and slightly charred around the edges.
Finish and serve: Remove from the oven. Immediately squeeze fresh lime juice over the shrimp and veggies and sprinkle with chopped cilantro. Serve hot with warm tortillas and optional toppings like sour cream, avocado, black beans, or rice.