In a large pan, heat 1 tbsp of butter and 1 tbsp olive oil over medium-high heat.
Once very hot, add the 1/2 of the chicken and cook until browned (split chicken amount to keep the pan hot and perfectly achieve a golden crust).
Once the chicken pieces are golden brown on one side, use a spatula to flip and cook on the other side.
Remove to a separate bowl or plate.
Repeat process with remaining 1/2 chicken.
Using the same pan, heat 1 tbsp olive oil and 1 tbsp butter.
Sauté the chopped onions and minced garlic over medium heat until translucent.
Add the sliced mushrooms and cook for 5-6 minutes (with lid on).
Add the flour and mix with the mushrooms. This will thicken the sauce later on.
Add the chicken and juices back to the pan.
Add the tomato sauce (or diced tomatoes), mustard, ketchup and cook for about 5 minutes.
Add the heavy cream, adjust salt and pepper. Simmer uncovered for about 5-6 minutes until the sauce has thickened.