1. In a small saucepan, melt 2 tablespoons butter.  Add the onions and simmer on low until caramelized.  This could be 15 min up to 20 minutes. Stir often as the smaller the onions are cut the easier they are to burn.
2. Add the remaining 4 Tbsp butter, garlic and thyme.  Let the butter melt.  Add the brown sugar.  Take off the heat.
3. Peel the sweet potatoes and slice them on a mandolin on the highest setting (1/4″).  You will need at least 12 slices per muffin tin (144 or more total)
4. Add the melted butter/onion/garlic and thyme to the potatoes and combine using your hands.  Break up any potatoes that are stuck together.  Add the shredded gruyere cheese and parmesan cheese.  Again, stir using your hands.
5. Spray a muffin tin with cooking spray and start layering the potatoes in each cup.  Put the biggest slices on the bottom and work your way around ending with the smallest ones on top.  This will take at least 12 slices, don’t worry if they are higher than the muffin tin.  They will shrink at least 25% as they cook down. Tuck any cheese that has fallen on the edges inside.  Use a butter knife to stack them up straight.  The more evenly they are stacked they better they will look on the plate.
6. Bake in a preheated oven at 375° F for 25-30 minutes.  They are done when you insert a knife and it touches the bottom with a little resistance.  If you cook them too long they will become mushy.
7. Let cool 5 minutes and transfer to a plate.

WHAT TO SERVE WITH THIS:
WEEKNIGHT FLANK STEAK
GRILLED CHICKEN