Heat the oil in a large skillet on medium-high heat. When the oil is hot add the cabbage and mushrooms. Cook for 4 minutes or until the mushrooms lose their water and begin to brown. Add the spinach, vinegar, and salt and pepper and cook until the vinegar is reduced to a thick glaze. Approximately 3 minutes. Remove from heat.

Spread a layer of butter on one side of each slice of bread and place butter side down in a hot pan (medium-high heat so they don't burn). Spoon the cabbage mixture on top of two of the slices of bread, divide equally. Sprinkle the vegan cheese on top (optional) and place the second piece of bread on top of the sandwich. They are done when the buttered sides are slightly brown and crispy.