In a 12 inch non-stick skillet over medium heat melt butter with oil and sauté mushrooms and onions until tender, remove from heat.
In a 2-quart saucepan over medium high heat, add the olive oil and butter. Once the oil is hot and the butter is melted, add the orzo and stir until lightly toasted, about 3 minutes. Add the garlic and cook for 30 seconds, stirring. Add the wine, stir, and cook to deglaze the pan.
Once the wine has nearly evaporated, add the mushrooms, onions and 1 cup of the stock, stirring constantly until the stock is absorbed by the orzo, about 10 minutes, repeat 1 cup of stock at a time stirring constantly and add the salt, and white pepper with last cup. When the last cup of stock is absorbed stir in the butter and adjust the seasoning, to taste.