Heat oil and butter in large skillet over medium heat until butter is melted. Add onion, garlic, rosemary, thyme and nutmeg; season with salt and pepper. Reduce heat to low; add snails and parsley. Cook 30 minutes, stirring occasionally.

Preheat oven to 350°F. Remove stems from mushrooms and discard.

Arrange mushroom caps upside down in 2-inch-deep baking dish; place 1 snail from garlic mixture in each mushroom cap. Pour garlic mixture over snails; cover with foil and bake 30 minutes.

Meanwhile, remove crusts from bread slices. Toast each slice and cut diagonally into 4 triangles. Serve with escargots.