For caramelized onions, heat a skillet over medium-low to medium heat and melt butter. When it foams, add the onions and season with bay, salt, white and fine black pepper, and thyme and cook slowly 35-40 minutes to deep caramel and very soft and sweet. Add consommé or stock and stir a minute or so and remove from heat.
Place potatoes and garlic in a pot and cover with water and bring to a boil. Add salt to boiling water and cook potatoes to fork-tender, 15-20 minutes. Drain potatoes and garlic and place in food mill or potato ricer. Mill or rice the potatoes and garlic and add milk and cheese to combine. Season with salt and white or fine black pepper and freshly grated nutmeg to taste, then stir in the caramelized onions.
Serve potatoes while hot or, to make ahead, cool to room temp and store in fridge.