Begin by thinly slicing your yellow onions. In a large Dutch oven, melt a generous amount of salted butter over medium heat. Add the sliced onions along with a pinch of kosher salt, black pepper, and a splash of red wine vinegar. Sauté for about 5 minutes until they soften.

Continue cooking the onions for 40-50 minutes, stirring frequently to prevent sticking or burning, until they have a deep caramel hue and silky texture.

While your onions are caramelizing, slice a French baguette into thick pieces. Butter each slice generously and sprinkle with garlic salt. Broil them for 2-3 minutes until golden brown and crispy, keeping an eye to prevent burning.

Once the onions are caramelized, pour in a cup of white wine and scrape up any brown bits stuck to the bottom of the pot. Let the wine simmer for about 2-3 minutes.

Add in beef broth and a bundle of fresh thyme. Stir to combine and bring to a boil. Once boiling, reduce heat and let simmer for another 10 minutes.

Remove the thyme bundle and ladle the soup into oven-safe bowls. Top with toasted croutons and sprinkle with Gruyere cheese. Broil for 2-3 minutes until cheese is bubbly and golden brown.

Carefully remove bowls from the oven and allow to cool slightly before serving. Enjoy your warm soup hot, optionally adding extra crunchy garlic croutons for dipping.