Heat olive oil and butter in a large skillet over medium heat.
Add the sliced onions and cook until caramelized, about 15-20 minutes.
Stir in the thyme and season with salt and pepper.
Add the Arborio rice and toast for 2-3 minutes.
Pour in the beef broth gradually, stirring continuously.
Bring to a simmer and reduce heat to low.
Cook for about 20 minutes until rice is creamy and tender.
Stir in the grated Gruyère cheese until melted and creamy.
Adjust seasoning as needed, adding more salt or pepper.
Garnish with fresh parsley before serving.