Spread butter on the outside of each slice, then layer cheese, sautéed mushrooms, and another layer of cheese between the bread, buttered sides facing out.
Grill it
Heat a pan over medium heat and grill for 2-3 minutes, until golden brown. Flip and cook the other side until crispy and the cheese is melty. Remove from heat and let it cool.
Pack it for lunch
Slice the cooled sandwich in half and pack it in a hard lunch container. Keep any sides like fruit and veggies in a separate compartment or container. Use an ice pack, if necessary.