Heat a large skillet over medium, add in half the butter and oil. Add in the chicken, salt and pepper and sauté for a few minutes on each side or until lightly golden. Remove and set side.
Add in the remaining butter and sauté the mushrooms for a few minutes, or until they have some good color. Sauté the garlic for 20 seconds, careful not to burn it! Add in the diced tomatoes with their liquid/sauce and the spinach and stir until the spinach has wilted.
Stir in the heavy cream, mascarpone and parmesan cheese. Once combined, add the chicken back in and simmer on medium low for 8-10 minutes (partially covered) or until the chicken is cooked through.
Add the fresh basil and serve! Store leftovers in the fridge for a few days or freeze.