Place potatoes in a saucepan and cover with water; bring to a boil. Reduce heat; cover and cook for 20-25 minutes or until tender. Drain. Mash potatoes with butter and onion until small lumps of potato remain; set aside.
In a bowl, beat ricotta cheese, sour cream, salt, garlic powder, rosemary and pepper until smooth. In a small bowl, beat egg whites until frothy; fold into cheese mixture. Fold into potato mixture.
Generously coat muffin cups with cooking spray; evenly sprinkle muffin cups with bread crumbs. Fill with potato mixture; smooth tops.
Bake, uncovered, at 425° for 27-30 minutes or until edges of potatoes are lightly browned. Cool for 15 minutes. Loosen timbales from sides of muffins cups; invert onto a baking sheet to remove.