Heat olive oil in a large pot over medium heat.
Add sliced onions and sauté until caramelized, about 15-20 minutes.
Stir in Arborio rice and cook for 2 minutes to toast.
Add thyme, bay leaf, salt, and pepper.
Gradually pour in the broth, one cup at a time, stirring frequently.
Cook until rice is al dente, about 18-20 minutes.
Remove bay leaf and adjust seasoning if needed.
Stir in half of the Gruyère cheese until melted.
Serve in bowls, topped with the remaining cheese.
Garnish with fresh parsley and serve warm.