1/4 cup mayonnaise 1 tablespoon Dijon mustard 1 tablespoon drained prepared horseradish Burger: 8 ounces bulk Italian sausage Kosher salt and freshly ground black pepper 1 tablespoon unsalted butter 1/2 cup shredded Cheddar (about 2 ounces) 4 English muffins, regular, sandwich-size or corn, split and toasted 1 vine-ripe tomato, thickly sliced into 4 piece 1 cup peppery greens, such as baby arugula, mustard greens or watercre 4 large egg