Bring the beef broth to a simmer in a medium saucepan over medium heat. Keep it warm while preparing the onions and rice.
In a large skillet, heat the olive oil and butter over medium heat. Once melted, add the sliced onions.
Sprinkle the sugar over the onions and cook, stirring occasionally, until they are caramelized, about 15-20 minutes, until golden brown and soft.
Add salt, black pepper, and dried thyme to the caramelized onions, stirring to combine.
Add the uncooked rice to the skillet with the onions and stir to coat the rice in the onion mixture.
Pour in the warm beef broth, stir well, and bring the mixture to a boil.
Once boiling, reduce the heat to low, cover the skillet, and simmer for 15-20 minutes or until the rice is tender and has absorbed most of the broth.
Once the rice is cooked, remove from heat and stir in half of the grated Gruyère cheese until melted and combined.
Serve hot, topped with the remaining Gruyère cheese and garnished with fresh parsley if desired.