Marinate the chicken: Dice the chicken breast, then mix with cornstarch, salt, pepper, dried oregano, and chilli pepper. Marinate for 5 minutes.
Prepare the honey mustard dressing: In a small bowl, mix together honey, mustard, mayonnaise, and lemon juice. Set aside.
Cook the chicken: In a pan skillet, heat some oil over medium-high heat, then sear the chicken breasts until golden-brown (about 2 minutes per side).
Make the sandwich: Spread the honey mustard dressing on 1 piece of bread, then place a piece of cheese, cooked chicken, and sliced avocado on top. Close the sandwich.
Toast the sandwich: Melt some butter in a pan over medium heat. Place the sandwich in the pan and cook for about 2-3 minutes per side, pressing down lightly, until the bread is golden and cheese is melted.
Serve: Slice in half and enjoy while hot and gooey!