Season the chicken breasts strips with 1 tablespoon of the fajita mix.: The spices should cling to the surface of the chicken , releasing fragrance as they hit the hot skillet. You want a light, even coating so each strip picks up flavor without being overwhelmed. When the seasoned meat hits the pan the scent of cumin and chili lifts immediately, which is a good signal your skillet is ready. A common mistake is over seasoning at this stage, which can mask the peppers and cheese, so use the measured tablespoon and save the remaining mix for the vegetables.
In a large skillet, heat 1 tablespoon of the olive oil and add the chicken to it. Cook the chicken for 5 to 10 minutes until it's fully cooked and no longer pink, and it's slightly charred, but not burned.: As the oil shimmers, lay the chicken strips down so they sizzle on contact, creating a golden sear. You should hear a steady sizzle and see the edges turn opaque; this means the Maillard reaction is working to build flavor. Cook until the chicken is no longer pink and has a slight char, which gives a smoky note. A pitfall here is overcrowding the pan, which steams rather than sears the meat, so give the strips room and work in batches if needed.
Remove chicken from skillet, and wipe the skillet clean.: Lifting the chicken off at the right moment keeps it juicy and prevents overcooking. After removing it, quickly wipe the skillet so residual browned bits do not burn when you cook the vegetables. Those browned bits can add flavor, but if left they may scorch and become bitter. Take care not to leave the skillet wet, as trapped moisture can prevent the peppers and onions from caramelizing.
Add another tbsp of olive oil to the skillet and heat it up. Add the onion and cook for a couple minutes until it’s slightly translucent. Add the peppers and the fajita mix and mix well. Cook for a couple more minutes so the peppers cook slightly. Add the chicken to the skillet, mix with peppers and onions and cook for a minute.: When the new oil heats, the onions will begin to sing with sweet aroma as they soften and release moisture. Adding the peppers and the remaining fajita seasoning ensures the flavors marry, and the sizzling melds their scents. Reintroducing the chicken allows the meat to absorb the pepper and onion juices, creating cohesion. Watch for softened edges and a slight char on the vegetables, and avoid overcooking which makes them floppy instead of pleasantly tender crisp.
In a clean skillet, add a bit of the butter and melt. Add a tortilla and move it around until it's buttery. Add a little bit of the chicken and onion mixture on top of the tortilla, and about 1/4 cup of cheese, depending on how you like it. Add another tortilla on top and pat it down.: The butter should foam gently and give off a nutty aroma as it melts, which helps the tortilla brown evenly. Moving the tortilla around coats it so no spot remains dry. Layer the filling in the center so the cheese melts into the chicken and veggies, and the heat seals the two tortillas together. Pressing down helps the ingredients compact into a cohesive wedge. A typical misstep is overfilling, which prevents proper toasting and makes flipping messy, so keep portions moderate.
Now flip it over and cook on the other side until the other side is nice and golden. Remove from skillet and cut into quarters. Repeat these steps with the remaining tortillas. Serve this with some sour cream and salsa.: When you flip, listen for a firm sizzle and watch for a deepening golden color around the edges, that tells you the cheese is melting and the tortilla is crisping. The aroma will be buttery, tangy from the cheese, and savory from the filling. Cutting into quarters should reveal stretchy strands of melted Tex Mex cheese and a tidy layered interior. Serve while hot so the contrast between crisp exterior and molten interior is at its best. Avoid sitting too long before serving or the tortillas will soften and lose their satisfying crunch.