Grilled Cheesy Veggie Melts

Name: Grilled Cheesy Veggie Melts
Ingredients: 1/4 cup brown mustard; 2 tablespoons mayonnaise; 1 tablespoon drained prepared horseradish; 2 cups loosely packed baby spinach; 1 1/2 cups very finely sliced green cabbage; 1/2 cup grated carrot; 1 1/2 teaspoons extra-virgin olive oil; 1 1/2 teaspoons white wine vinegar; Kosher salt and freshly ground black pepper; 8 slices sourdough bread; 12 ounces sliced fontina cheese; 3 tablespoons unsalted butter