Preheat oven to 450 degrees.
Arrange potatoes and onions in large shallow roasting pan lined with foil.
Combine oil, butter, garlic, salt and pepper in a small bowl.
Drizzle over potatoes and onions and toss well to combine.
Bake 30 minutes.
Stir and bake 10 minutes more.
Add herbs; toss well.
Continue baking 10 to 15 minutes or until vegetables are tender and browned.