2 tablespoons extra-virgin olive oil
5 cloves garlic, minced
3 large onions, thinly sliced with the grain
1 tablespoon fresh (or 1 teaspoon dried) thyme leaves
1/4 cup sherry vinegar
2 11.2-ounce bottles stout beer, such as Guinness
6 cups beef broth
1 baguette, cut into 1/2-inch-thick slices
1/2 pound Irish cheddar, such as Kerrygold Kilaree, thinly sliced
In a large skillet, heat olive oil over high heat. Add garlic and cook 2 minutes. Add onions and cook for 40 minutes, stirring occasionally, until the onions are golden brown. Stir in thyme, vinegar, and beer. Cook until beer reduces by half. Pour in beef stock. Bring to a simmer and cook for 10 more minutes.
Preheat broiler oven setting. Ladle soup into individual ovenproof soup bowls. Top with bread slices and sliced cheddar. Broil until cheese melts and begins to brown. Serve piping hot.