01 -
Sprinkle salt and black pepper evenly over both sides of the chicken breasts.
02 -
Warm olive oil in a skillet over medium-high heat and cook chicken for 6-7 minutes per side until fully cooked and internal color is no longer pink. Remove and allow to rest before slicing thinly.
03 -
Slice the avocado and cheese; set aside for assembly.
04 -
Evenly coat each slice of bread with mayonnaise and Dijon mustard.
05 -
On two bread slices, layer sliced chicken, avocado, cheese, red onion, and tomato; cover each with the remaining bread slices, condiment side down.
06 -
Heat butter or olive oil in a skillet over medium heat. Place sandwiches in pan and cook 3-4 minutes per side until bread is golden and cheese melts, pressing gently with a spatula for even melting.
07 -
Remove sandwiches from heat, allow to rest briefly, slice in half, and serve warm.