Step 1: Mix the Fajita Seasoning — In a small bowl, stir together the chili powder, cumin, smoked paprika, garlic powder, onion powder, oregano, salt, black pepper, and cayenne (if using).

Step 2: Layer Everything in the Slow Cooker — Place the chicken breasts in the bottom of a 6-quart slow cooker. Scatter the sliced bell peppers and onion over and around the chicken. Sprinkle the garlic and the entire batch of fajita seasoning evenly over everything. Drizzle with the olive oil and lime juice. Spread the drained diced tomatoes with green chilies over the top.

Step 3: Cook Low and Slow — Cover and cook on LOW for 4–5 hours or on HIGH for 2–2½ hours, until the chicken is very tender and shreds easily with a fork (internal temperature at least 165°F / 74°C). Don't overcook.

Step 4: Shred the Chicken — Remove the chicken breasts to a cutting board and shred with two forks (or slice into strips). Return the shredded chicken to the slow cooker and stir it into the peppers, onions, and juices. Let it rest on warm for 10 minutes.

Step 5: Warm the Tortillas and Assemble — Warm the flour tortillas in a dry skillet over medium heat for 30 seconds per side (or wrap in foil and warm in a 350°F / 175°C oven for 10 minutes). Set out the toppings and let everyone build their own fajitas.

Step 6: Serve — Spoon the fajita chicken and peppers into warm tortillas and top with sour cream, salsa, guacamole, lettuce, cheese, and cilantro. Serve with lime wedges.