In a large pot or Dutch oven, melt the unsalted butter over medium heat. Once melted, add the thinly sliced onions. Cook, stirring frequently, for about 10-15 minutes, until the onions are caramelized and golden brown.
Add the Arborio rice to the pot and stir to combine with the onions, cooking for an additional 2-3 minutes, allowing the rice to toast slightly.
Pour in the beef broth and add the dried thyme, bay leaf, salt, and pepper. Stir well and bring the mixture to a simmer.
Reduce the heat to low, cover the pot, and let it cook for about 20 minutes or until the rice is tender and has absorbed most of the liquid. Remove the bay leaf.
Preheat your oven to 400°F (200°C).
Divide the rice mixture into oven-safe dishes, then top each with grated Gruyère cheese. Bake in the preheated oven for 10-15 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and allow to cool for a couple of minutes. Garnish with chopped fresh parsley before serving.