If you want to serve this for company, prepare it in the morning, refrigerate, and let sit on your counter for 30 minutes before baking.
Serve with risotto, or pasta, or even roasted potatoes and a green salad.
4 T avocado or extra-light olive oil, divided
In a large skillet, heat 2 tablespoons of the oil over medium-high heat.
Add the mushrooms and cook until golden, about 10 minutes, stirring often.
Lower heat to medium and add the onion and cook another 2 minutes.
Add garlic and cook another minute.
Spoon half of this mixture into a baking dish and the rest in a bowl and set aside.
Cut each chicken breast in half lengthwise.
In the same skillet, add 1 tablespoon of oil and brown half of the chicken on both sides until golden, about 4 minutes per side.
Place cooked chicken on top of mushroom mixture in baking dish.
Add the final tablespoon of oil to the skillet and brown the rest of the chicken on both sides until golden, about 4 minutes per side.
Spoon the rest of the mushroom mixture on top of the chicken.
Preheat oven to 350 degrees F.
Add the wine and chicken stock to the skillet and bring to a simmer.
Add the butter and simmer 5 minutes, stirring occasionally.
Season to taste with salt and pepper and pour over the chicken in the baking dish.
Combine the cheeses with the scallions and sprinkle over chicken.
Bake uncovered for 30 minutes, until cheese is melted and bubbly.