Preheat your oven to 350°F (180°C) and line a baking tray with parchment paper.
In a large skillet, add the olive oil and unsalted butter over medium heat. Once melted, add the finely diced yellow onion and sauté until soft and translucent, about 5 minutes.
Stir in the minced garlic and sliced cremini mushrooms. Cook until the mushrooms are browned and moisture has evaporated, approximately 8 minutes.
Sprinkle in fresh thyme leaves along with salt and black pepper to taste. If using, pour in the dry white grape juice and cook until any liquid is absorbed. Reduce heat to keep warm.
Stir in cream cheese, finely grated Parmesan cheese, and optionally Dijon mustard until combined. Remove from heat and gently fold in the grated Gruyère cheese.
