How to elevate the classic grilled cheese? How about filling thick slices of brioche with three cheeses, fresh herbs, a splash of prosecco and a spoonful of Dijon mustard? Add a jammy onion spread inspired by the flavors of French onion soup and you've got one unbeatable sandwich!
Heat 1 tablespoon butter in a medium skillet over medium heat. Add the onions and sprinkle with the thyme, 1/2 teaspoon salt and several grinds of pepper. Cook, stirring occasionally, until the onions are browned and tender, about 20 minutes. Turn off the heat and stir in 1 tablespoon of the Dijon mustard and 1 tablespoon water to coat the onions. Transfer to a bowl and set aside.
Heat a griddle over medium heat.
Buzz the Cheddar, Gruyere, Robiola, prosecco, parsley, garlic and remaining 1 teaspoon Dijon mustard in a food processor until combined but still chunky and spreadable. Evenly spread the cheese mixture over one side of each slice of bread. Top 2 slices with the onion mixture and then sandwich the onions with the other 2 slices, cheese-side in. Spread the remaining butter on the outside of each sandwich and season with salt and pepper. Griddle the sandwiches over medium heat until golden brown on the first side, about 5 minutes. Flip and continue to cook until the cheese is melted and the exteriors are crispy, about 5 minutes more.