Heat a large skillet over medium, add in half the butter and oil. Add in the chicken, half the salt and pepper, then sauté for a few minutes on each side or until lightly golden. Remove and set side.
Add in the remaining butter, then 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, tomato paste and the spinach and stir until the spinach has wilted.
Stir in the heavy cream, mascarpone, parmesan cheese and remaining salt and pepper. Once combined, add the chicken back in and simmer on medium low for 8-10 minutes or until the chicken is cooked through and the sauce has thickened.
Add the fresh basil and serve! Store leftovers in the fridge for a few days or freeze.
