This delectable classic is made with caramelized onions for a sweetness that contrasts with the savory-saltiness of the broth and the cheese garnish. The length of time needed for the browning of the onions is critical; it will make the difference between an insipid soup and a transcendent one.
4 tablespoons unsalted butter
4 large Spanish onions, about 4 pounds, thinly sliced, or a mix of red onions and Spanish onions
Freshly ground black pepper
1 bay leaf ½ teaspoon dried thyme, or 1 teaspoon fresh thyme leaves
1 cup dry white wine
2 cups beef stock or broth
4 cups chicken stock or broth
2 tablespoons chopped fresh parsley
8 to 12 slices French bread, toasted
1 cup grated Gruyère cheese
Melt the butter in a 4-quart heavy-bottomed casserole or Dutch oven, or a large skillet over medium-low heat. Add the sliced onions, season with sea salt, toss well, and cook until the onions are well browned and caramelized, stirring often, 45 minutes to 1 hour. Lower the heat as the onions brown so as not to burn them. Add the pepper, bay leaf, thyme, and white wine and bring to a boil, stirring occasionally. Add the beef and chicken stock and bring back to a boil. Lower the heat and simmer the soup for 45 minutes.
Preheat the broiler.
Remove and discard the bay leaf and stir in the parsley; taste for sea salt and pepper. Add more if needed, keeping in mind that the cheese is salty. Place a crouton in each of 4 to 6 ovenproof bowls. Add a portion of soup and top with a portion of cheese. Place the bowls on a sturdy cookie sheet, preferably one with sides, or a roasting pan. Broil the soup about 4 inches from the heat until the cheese is browned and bubbly. Let the soup settle and cool for a few minutes before serving, otherwise you might burn the roof of your mouth!