Melt butter in a large cast ion skillet (or oven-safe skillet*) over medium heat. Add onions and mushrooms to the skillet, cook for 3 minutes or until the onions start to brown.
Add garlic and cook for 30 seconds, or until fragrant. Add spinach and cook till spinach wilts, about 1 minute. Salt and pepper to taste ( I do ½ teaspoon salt and ¼ teaspoon pepper)
Move vegetables to a bowl with goat cheese, mix until the goat cheese melts, and coats the vegetables.
Salt and pepper each side of the chicken breast.
Add 1 tablespoon of butter to the skillet. Once it has melted add the chicken. Cook on each side till it browns. About 2 minutes for each side
Top chicken with the vegetable mixture and sprinkle parmesan on top.
Bake for 12-15 minutes, or until the chicken has reached a temperature of 165F.
Notes
If you don't have an oven-safe skillet, do steps 1-6 in a large skillet, place the chicken on a sheet pan, top it with the vegetable mixture and sprinkle parmesan on top. Bake for 15-20 minutes.