Remove the lid, increase the heat to medium. Stir in the salt and sugar. Cook for 30-40 minutes until the onions are golden brown. Stir frequently to keep the onions from burning.

Sprinkle the gluten-free flour over the onions and stir until combined. Pour in the Vermouth and continue to stir until the liquid reduces. This step is very fast. Pour in about 1 cup of beef stock and stir into the onion mixture. Continue to stir using the beef stock to loosen the flour mixture. As you stir, you'll incorporate the flour into the liquid to avoid clumps. Once thoroughly incorporated, pour in the remaining beef stock and stir.

Bring to a boil and reduce the heat. Simmer for 30 minutes.

Preheat the oven to broil. Place the slices of gluten-free bread on a baking sheet. Top each slice of bread with provolone cheese. Heat for 2-3 minutes until the cheese is melted and bubbly.

Remove from the oven and serve.