Heat skillet with butter over medium heat, add onions, season with salt and pepper and ground thyme, bay leaf and cook until very soft and sweet and lightly caramelized, 20 minutes. Remove bay leaf.

While onions cook, place potatoes in pot and cover with water, bring to boil, salt water and cook to tender, 12 to 15 minutes, drain and return to hot pot. Mash the potatoes with cream, season with salt and pepper and mash in Parm.

Arrange mashed potatoes in individual gratin dishes or 1 shallow casserole. Top potatoes with onions and Gruyere cheese. Brown the sweet onion and potato gratins under hot broiler 2 minutes until bubbly.