Vegetarian Reuben

Name: Vegetarian Reuben
Ingredients: 4 slices rye or pumpernickel bread; 4 slices Swiss cheese (lacey, baby, or regular); 10 to 15 button mushrooms (cleaned and sliced); 1/4 head red cabbage (sliced); 2 cloves garlic (minced); 3 tablespoons mayonnaise; 2 tablespoons Dijon mustard; 2 tablespoons dill pickle relish; 2 tablespoons prepared horseradish; 1/4 cup apple cider vinegar; 1 pinch sea salt and freshly ground black pepper; 1 serving unsalted butter, for spreading; 1 splash Olive oil