Name: Alex Guarnaschelli's Burger With Cheese Sauce
Yield: 4
Ingredients: 2 medium red onions; cut into 1/2-inch rounds 3 tbsp unsalted butter Kosher salt 1 tbsp red wine vinegar 2 large cloves garlic; minced 1/2 cup whole milk 1/2 cup heavy cream 1 tbsp grainy mustard 2 cups grated gruyere cheese 1/4 cup finely grated Parmesan cheese 1 tsp paprika Worcestershire sauce hot sauce 1 1/2 lbs ground beef (preferably chuck and not too lean) 4 seeded hamburger rolls; halved 1 small Serrano chili; cut into very fine slices