CHICKEN CASSEROLE WITH ONION GRATIN

3 boneless, skinless chicken breasts
2 tsp (10 mL) garlic
1 tsp (5 mL) dried oregano
Crushed red pepper flakes, optional to taste
Pinch each salt and fresh cracked pepper
1 cup (250 mL) Gruyere cheese
2 tbsp (30 mL) butter
5 large onions, thinly sliced
​​3 cloves garlic, minced
1/4 to 1/2 cup (60 to 125 mL) chicken broth
Chopped fresh parsley, for garnish
Preheat your oven to 400°F. Lightly grease a 9×12-inch baking dish and set aside.
Melt the butter in a large skillet over medium-high heat.