Cook potatoes in boiling salt water until tender. Drain and set aside. In saucepan melt butter. Saute onion in butter until soft, stir in flour, thyme, salt and peppr. Gradually add cream, stirringconstantly until sauce thickens. Remove from heat; let cool slightly. Add mustard and mayo, stir until smooth. Place potatoes in 1-1/2qt. Casserole dish. Cover with sauce. Bake @ 350 for 30 minutes. Before serving sprinkle edges of caserole with crumbled bacon.