Preheat oven to 425°. In a Dutch oven, heat butter and 2 tablespoons oil over medium-high heat. Add mushrooms; cook and stir 4-6 minutes or until browned. Remove and set aside.
In the same pan, heat remaining oil. Add onion; cook and stir 3-4 minutes or until tender. Add garlic; cook 1 minute longer. Add wine, stirring to loosen browned bits from pan. Bring to a boil; cook 2-3 minutes or until wine is almost evaporated. Stir in stock, parsley, thyme, salt, pepper and mushrooms. Bring to a boil; cook 3-4 minutes longer or until liquid is reduced by half.
In a small bowl, mix flour and softened butter until blended; stir into mushroom mixture until thickened. Transfer to a greased 11×7-in. baking dish.
Meanwhile, place potatoes in a large saucepan; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, 10-15 minutes or until tender. Drain; return to pan. Mash potatoes, gradually adding milk, butter, garlic powder, salt and pepper.
Spread potatoes over mushroom mixture; sprinkle with cheese. Bake 15-20 minutes or until heated through. If desired, top with additional thyme or parsley.