Heat 1 tablespoon butter in heavy small skillet over medium-low heat. Add onion and sage and saute 4 minutes. Add garlic and cook 30 seconds. Transfer onion mixture to bowl. Cool slightly. Add remaining butter and stir to combine. Mix in parsley. Season to taste with salt and pepper. Place butter on piece of plastic wrap. Roll up into log and refrigerate. (Butter can be prepared up to 2 days ahead.)