Preheat oven to 450 degrees.

Cook onions in a 12" nonstick skillet over high heat, stirring constantly, until soft and caramelized, about 15 minutes. Add butter and toss onions to coat.

Remove pan from heat and stir in brandy.

Return pan to heat and continue cooking until brandy is absorbed, about 30 seconds.

Add stock and bring to a simmer. Cook until soup is reduced to 4 cups, about 5 minutes. Season soup to taste with salt and pepper.

Place ramekins on a rimmed baking sheet. Divide soup among ramekins. Top each ramekin with 1-2 slices of bread. Sprinkle 1/4 cup cheese over each.

Transfer baking sheet with ramekins to oven and bake until cheese is bubbly and browned in spots, about 4 minutes.