In a large, heavy-bottomed soup pot, melt butter over medium heat and add onions. COVER and cook 10 minutes.

UNCOVER, add thyme, bay leaves, salt, black pepper and cook onions until they are very soft and deep golden brown, stirring occasionally, about 45 minutes. I usually start at medium heat and then when I see the onions start to color, I drop to medium-low and then to low heat. This is the key step in this recipe. If the onions don’t get golden, the soup won’t have as much flavor. But you also don’t want to burn the onions because then the soup will taste bitter.

Stir in the wine and Cognac, scraping the bottom of the pot.

Stir in the stock and bring to a boil. Lower heat to a simmer, and cook uncovered, stirring occasionally, about 30 minutes longer. Remove the bay leaves and thyme sprigs. Taste for seasoning. Keep in mind that if you are adding cheese, you will be adding extra saltiness.

In the meantime, preheat the oven to 400 degrees, arrange the baguette slices or bread pieces on a baking sheet in one layer and bake until lightly golden, about 10 minutes.

Preheat the broiler to high and position the oven rack 6 inches from the heat. Either top each piece of toast with a handful of grated gruyere and broil until melted. Divide the soup between 6 bowls and top each soup bowl with a piece of cheese toast. OR place 6 oven-proof soup bowls or crocks on a rimmed baking sheet and fill each with soup. Top with the toasted bread and then the cheese. Broil a few minutes until cheese has melted.