In a shallow bowl, mix all the spice mix ingredients and set aside.
Slightly pound down the chicken to an even thickness, and sprinkle the tops with spice mix; set aside.
In a large skillet over medium heat, add oil. When hot, place the chicken, seasoned side down, and add more spice mix on top. Cook the meat until the internal temperature reaches 160ºF.
Transfer onto a cutting board and wrap tightly with foil. Let the breasts rest for 7 to 10 minutes, before slicing against them grain into ½-inch strips.
Meanwhile, in the same skillet, add onion slices and stir to coat them with fat left behind; season with salt. Sauté for 1 to 1:30 minutes.
Add peppers, a little bit of oil if needed, and the remaining spice mix. Sauté until the veggies are softened, about 5 minutes.
Return the chicken strips and gently toss until the meat is heated through, 2 minutes at the most.
Stir in lime juice.
Spoon mixture into warm tortillas and garnish with your favorite toppings.