Tuscan Chicken

1 lb boneless skinless chicken
Oil and butter for frying
Salt, pepper, oregano
Sauce:
1 cup diced onion
3 cloves minced garlic
1/2 cup chopped mushrooms
2 cups cherry tomatoes halved
1-1/2 cup cream or half and half
1/2 cup grated parmesan cheese
3 cups baby spinach or chopped spinach