Preheat oven to 200℉
Season the boneless skinless chicken breasts on both sides with the garlic salt and pepper.
In a large sauté pan on medium-high heat 2 Tablespoons of the olive oil and when it is hot add 2 Tablespoons of the butter and swirl around.
Add the garlic to the pan and sauté until it's fragrant. Do not burn.
Place both breasts in the pan and brown on one side about 5 minutes and then turn to the other side and brown another 5 minutes. Transfer the chicken to a heat proof plate. Loosely cover with aluminum foil and place in the preheated oven to keep warm.
In the same pan, add the rest of the oil and stir in the mushrooms. Cook for another 5 minutes stirring occasionally. When the mushrooms, turn golden.
Add the baby spinach to the mushrooms and mix.
Add the chicken stock to the pan. Salt and pepper the vegetables. When the spinach begins to wilt, add the grape tomatoes. When the tomatoes soften, push the vegetables to the side and nestle the chicken breasts in the center.
Place the sauté pan in the oven for about 10 minutes. When the center of the chicken reaches 165℉ the chicken is ready to serve.
Place each chicken on a dinner plate and serve with the vegetables. Sprinkle with Parmesan cheese.