Brush butter on outer side of each slice of bread.
Add half of the beans to a bowl and mash gently with a potato masher or the back of a fork. Stir in remaining whole beans. Add in the mayonnaise and vegetables; mix together.
Spread evenly on 4 slices of bread. Layer the Swiss cheese on top with remaining slices of bread.
Grill on both sides.
Let rest several minutes so cheese does not run when cut. Cut and serve.