Place onion, peppers, and chicken in 9×13 glass baking dish. Drizzle and coat with olive oil. Sprinkle taco seasoning over dish and toss to coat. Roast in the oven for 25-35 minutes, stirring once halfway. Top with lime juice.
Assemble grilled cheeses by layering bread slice, cheese, chicken & vegetables, another layer of cheese, and top slice of bread. Grill in a panini maker. (Or, in a skillet like traditional grilled cheese.) Serve with guacamole.
Recipe by Rachel Schultz at https://rachelschultz.com/2015/01/20/chicken-fajita-grilled-cheese/