1. In a large pot, melt butter with oil over medium heat.
2. Add onions and mushrooms and cook until tender, about 10 - 15 minutes.
3. Add flour, salt and pepper to taste, and stir constantly for a minute until flour is absorbed.
4. Add stock, stirring until smooth.
5. Stir in half-and-half and reduce to low heat.
6. simmer for 10 minutes and serve hot.