Season the chicken breasts with salt, pepper, and garlic powder and let them marinate well for at least an hour.

Heat the oil in a non-stick grill pan and grill the chicken breasts over medium heat for a few minutes on each side. Be careful not to grill them too short because they'll be raw inside or too long because they'll just get dry.

Since I didn't have a non-stick grill pan, I used a regular deep non-stick pan which works just as well. Once the breasts are done, place them on a plate.

Slice the mushrooms and cut the onion in half, and then each half in strips.

First, put the onion in the same oil in which you grilled the chicken breasts. Turn up the heat a bit, season it with a pinch of salt, stir nicely, and let it saute for around 5 minutes.

Once it's done, take it out in a bowl, add a bit of oil into the pan if needed, and put the mushrooms inside. On medium heat, saute them for 8 minutes, and once they become brownish, they are ready!

Heat the oven to 350 °F (180 °C). Cover the casserole dish with baking paper and place the chicken breasts. On top of each chicken breast, put some onion, then mushrooms, and a slice of Jack cheese on the top.

It will only take 5 minutes in the oven for the cheese to melt, and your delicious meal for 4 people is ready!