Slice the bell peppers into strips and the onion into thick slices. Set aside.
Mix the cumin, smoked paprika, chili powder, garlic powder, cayenne, and salt in a small bowl to create your fajita spice blend.
Pat the chicken breasts dry and slice them into thin strips. Toss the chicken with half of the spice blend until evenly coated.
Heat 2 tablespoons of olive oil in a large skillet over medium-high heat until shimmering and hot.
Add the sliced peppers to the skillet and cook, stirring occasionally, for 5-7 minutes until they begin to soften and develop deep char around the edges. They should smell smoky and sweet.
Push the peppers to one side of the skillet and add the chicken strips to the empty space. Cook for 3-4 minutes without moving, then stir to sear evenly; cook until golden brown and cooked through, about another 4-5 minutes.
Squeeze the lime juice over the skillet contents and toss everything gently to combine and brighten the flavors.
Remove the skillet from heat and let it rest for a minute. Serve the sizzling fajita mixture with warm tortillas, and garnish with your favorite toppings like avocado, cheese, or sour cream if desired.