This is a much less time-consuming version of this sandwich.
Sliced mushrooms
Sliced yellow onions
1 T butter and 1 T olive oil (or more if you are making several sandwiches)
Fontina cheese (I think Swiss and Provolone would be good too.)
Fresh baby spinach leaves
Foccacia bread (or other artisan bread), sliced
Saute mushrooms and onions in butter/oil.
Layer--bread, mushrooms/onions, spinach, cheese, bread. Brush exterior with olive oil. Grill on panini maker until cheese is melted and you have some good grill lines.
