Peel and thinly slice the Yellow Onion into half-moons so they soften and caramelize evenly.
In a large heavy pot, melt the Unsalted Butter with the Olive Oil over medium heat until foaming.
Add the sliced onions and a pinch of Fine Sea Salt, then cook gently, stirring often, until the onions are deep golden brown and sweet—about 30–40 minutes; reduce heat if they begin to burn.
Stir in the minced Garlic and cook for 30–60 seconds until fragrant.
Sprinkle the Plain Flour over the onions, stir to coat, and cook for 2–3 minutes to remove the raw flour taste.
Pour in the Dry White Wine to deglaze the pot, scraping up any browned bits from the bottom, and let it reduce by half.
Add the Beef Stock, a few sprigs of Thyme and the Bay Leaf, bring to a gentle simmer, and cook uncovered for 20–30 minutes to meld the flavors.
Taste and adjust seasoning with more Fine Sea Salt and freshly ground Black Pepper as needed; remove the bay leaf and thyme sprigs before serving.
While the soup simmers, slice the Baguette and toast the slices until crisp. Preheat your oven broiler (or set oven to high broil).
Ladle the hot soup into oven-safe bowls, place toasted baguette slices on top of each bowl, generously sprinkle grated Gruyère over the bread, and place under the broiler until the cheese is melted, bubbly, and golden (watch closely so it doesn’t burn).
Carefully remove the bowls from the oven (they will be hot) and let cool briefly before serving. Garnish with a small thyme sprig if you like.
