Place the potatoes in a large saucepan and cover with water; bring to a boil. Reduce heat; cover and cook for 20-25 minutes or until tender. Drain and place potatoes in a large bowl. Add the milk, butter and salt; mash until light and fluffy. Fold in the sour cream, ricotta cheese, egg and onions.
Generously coat six muffin cups with cooking spray. Fill with potato mixture; smooth tops. Bake at 375° for 20-25 minutes or until edges are lightly browned. Cool for 5 minutes. Carefully run a knife around the edge of each muffin cup; invert onto a baking sheet or serving platter.
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