01 - Warm olive oil and unsalted butter in a large pot or deep sauté pan over medium heat until melted.
02 - Add sliced yellow onions and a pinch of salt. Cook, stirring frequently, until onions achieve a deep golden brown coloration, approximately 20 to 25 minutes. Reduce heat as needed to prevent burning.
03 - Incorporate minced garlic and allow to cook for 1 minute until aromatics are released.
04 - Sprinkle all-purpose flour over the caramelized onions, stirring to coat. Continue cooking for 1 minute to eliminate raw flour taste.
05 - Pour vegetable broth slowly while scraping the pan to deglaze, ensuring all fond is lifted. Add dried thyme and bay leaf for seasoning.
06 - Bring contents to a simmer. Add dry pasta, stirring well to submerge and distribute. Cover and cook, stirring occasionally, until pasta is al dente and most liquid is absorbed, roughly 10–12 minutes.
07 - Remove the bay leaf. Reduce heat and stir in heavy cream, shredded Gruyère, and grated Parmesan until fully melted and a creamy texture is achieved.
08 - Adjust seasoning with salt and freshly ground black pepper to your taste.
09 - Serve hot, garnished with freshly chopped parsley and additional Gruyère or Parmesan cheese as desired.