Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 10-15 minutes or until tender. Drain and set aside.
In large saucepan, melt butter. Saute onions until tender; stir in the flour, thyme, salt and pepper until blended. Gradually add cream. Bring to a boil. Cook and stir for 2 minutes or until thickened. Remove from the heat; let cool slightly. Stir in the mayonnaise and mustard. Pour sauce over potatoes; transfer to a 1-1/2-qt. baking dish.
Bake, uncovered, at 350° for 30 minutes. Just before serving, sprinkle with crumbled bacon.