In a large deep saucepan over medium heat, add oil and 1 tablespoons butter. When melted and hot, add onions, stir, cover and cook for 3 minutes. Add mushrooms, stir well and reduce heat to medium-low, add remaining butter and a generous pinch sea salt; cook for 4 minutes. Half way though cooking, stir to prevent from sticking and crack black pepper; cover and return to cooking.
Add flour, stir to coat and cook for 1 ½ minutes. Pour in heavy cream and milk. Increase heat to medium-high, stir, add cooking sherry, basil, thyme and cayenne pepper. Combine the ingredients and bring to a boil.
Reduce heat to medium-low and simmer gently for 30 minutes, stirring often; taste and adjust if necessary. If the consistency of the soup is too thick, add about ¼ to 1/3 cup chicken or vegetable broth. Makes 6 servings.
Note: The amount of chopped mushrooms should equal to 10 cups.