In a large skillet, heat 1 tablespoon of oil over medium heat. Add 2 pounds of sliced yellow onions and cook slowly, stirring occasionally, for about 60 minutes until they reach a deep brown color. Deglaze with 1 cup of beef broth as needed.

Once the onions are perfectly caramelized, add 1 chopped garlic clove and ½ teaspoon of chopped thyme to the skillet. Sauté for about 1 minute, then pour in 1 teaspoon each of Worcestershire sauce, balsamic vinegar, and soy sauce. Stir to combine.

In a large pot, bring salted water to a boil. Add 3 pounds of peeled and cubed russet potatoes along with 4 peeled garlic cloves. Cook until fork-tender, about 15 to 20 minutes. Drain thoroughly.

Return the drained potatoes and garlic to the pot. Mash until smooth, then mix in the caramelized onion mixture, ¼ cup of melted butter, ¼ cup of cream, and 1 cup of shredded gruyere cheese. Stir until creamy and well combined.