Step 1

Place stock and wine in a medium pan over high heat. Reduce liquid by half; set aside.

Step 2

Bring a medium saucepan of water to a boil. Add onions; cook 10 minutes. Drain, and cool. Using a paring knife, remove skins.

Step 3

Place flour and reduced liquid in the bowl of a food processor. Pulse until combined.

Step 4

Transfer mixture to a small saucepan; set over medium heat. Whisk constantly until thickened, about 10 minutes. Remove from heat. Stir in 1 1/2 teaspoons thyme and the pepper. Cut off top of garlic head, and squeeze flesh into the sauce; stir to combine.

Step 5

Place onions and sauce in a 10-inch round baking dish. Sprinkle with Parmesan and remaining teaspoon thyme. Bake until golden on top, about 1 hour 15 minutes. Serve.