Step 1

Combine the mustard, mayonnaise and horseradish in a small bowl. Stir to combine and set aside.

Step 2

Combine the spinach, cabbage and carrot in a medium bowl. Drizzle with the olive oil and vinegar. Season with salt and pepper. Toss to combine.

Step 3

Lay the bread slices out on your work surface and spread with the mustard mixture. Top 4 of the slices with half of the cheese. Pile the vegetable mixture on top of each, making an even layer. Top with the remaining cheese, then the remaining bread, sauce-side down. Press in the sides of the sandwiches, if needed, to neaten them up.

Step 4

Heat a large 2-burner flattop griddle over medium-low heat. Melt the butter and brush all over the griddle. Lay the sandwiches on the griddle and toast until crisp on the underside, 3 to 4 minutes. Flip and toast the second side, until the cheese is melted and oozy, about 4 minutes more.

Step 5

Remove the sandwiches to a cutting board and slice in half with a serrated knife.