Creamy Mushroom Soup

What is required:

Half a litre of butter
1 small onion, minced
Olive oil 1/4 cup
1 pound of sliced mushrooms (any kind, including portobello, cremini, button, etc.)
Two garlic cloves, minced
Thyme, either half a teaspoon dry or one teaspoon fresh
Season to taste with salt and pepper
Boil 3 cups of chicken or vegetable stock.
1.5-quart measuring cup of heavy cream
1 tablespoon of all-purpose flour may be added for thickening purposes.