French Onion Soup Gratinée

Melt butter in a large pot over medium-high heat. Stir in red onions, sweet onions, and salt. Cook, stirring frequently, until onions are caramelized and almost syrupy, about 35 minutes.

Stir in chicken broth, beef broth, red wine, and Worcestershire sauce. Bundle parsley, thyme, and bay leaf with kitchen twine; add to the pot. Simmer over medium heat for 20 minutes, stirring occasionally. Remove and discard herb bundle. Reduce heat to low; stir in vinegar and season with salt and pepper. Cover soup and keep warm over low heat while you prepare the toast.

Set an oven rack about 6 inches from the heat source and preheat the oven's broiler. Arrange bread slices on a baking sheet and broil, turning once, until well toasted on both sides, about 3 minutes. Remove from heat; do not turn off the broiler.