Chicken Avocado Melt Sandwich

2 boneless skinless chicken breasts
Salt and pepper to taste
1 tablespoon olive oil
1 avocado, sliced
4 slices of your favorite cheese (Swiss, cheddar, or provolone)
4 slices of bread (sourdough, ciabatta, or your choice)
2 tablespoons mayonnaise
1 tablespoon Dijon mustard
1 small red onion, thinly sliced
1 tomato, sliced
Butter or olive oil for grilling the sandwiches
How to Make: