Melt butter in a large saucepan over medium heat.

Add sliced onions and cook, stirring occasionally, until deeply caramelized and dark brown, approximately 20 to 25 minutes.

Stir in the rinsed rice and cook for 2 minutes to toast the grains.

Incorporate the beef broth, Worcestershire sauce, thyme, salt, and pepper.

Bring the mixture to a boil, then reduce heat to low and cover with a tight-fitting lid.

Simmer for 18 minutes or until all liquid is absorbed and rice is tender.

Remove from heat and let stand, covered, for 5 minutes.

Fold in the shredded Gruyère cheese until melted and the texture is creamy.

Garnish with fresh parsley before serving.