Stir in rosemary, bay leaves, thyme, brown sugar, red grape juice, and balsamic vinegar. Let it simmer until thickened.
Spread softened butter on one side of each bread slice. Layer Gruyere, white cheddar, and onion jam on unbuttered sides. Top with another slice, buttered side up.
Cook the assembled sandwiches in the skillet for 4-5 minutes on each side until golden brown and cheese is melted.
Optional: Sprinkle freshly grated Parmesan on top after cooking. Cut sandwiches in half and serve warm.
Notes
You can add roasted garlic or mushrooms for extra flavor. Store leftovers in an airtight container, and reheat in a skillet.