Heat the vegan butter in a pan over medium heat. Add the onion and sauté for about 5 minutes until the onion becomes translucent and just begins to brown. Add in the mushrooms and garlic and continue to sauté until the mushrooms have decreased in size and browned up nicely. Remove from heat and set aside.
Butter one side of each slice of bread. Spoon a nice helping of the mushroom and onion mixture onto 3 slices of bread (buttered side down), cover with the nacho cheese, and top with a remaining slice of bread (butter side up). Repeat with the remaining sandwiches.
Heat a pan over medium heat, and grill each sandwich for a few minutes on each side, until golden brown. Serve while hot and gooey!