Add 1/2 the butter and mushrooms to a large skillet, over medium-high heat. Cook for about 5-6 minutes, until mushrooms become tender and brown, and have released most of their water. Remove from the heat, and reserve to the side.
In a medium-sized bowl, mash gigandes until a smooth paste is formed.
Evenly distribute and spread the other half of the butter on one side of each slice of bread.
For each sandwich, spread gigande paste on the unbuttered side of one slice of bread.
In a large skillet, over medium-high heat, cook two sandwiches at a time by placing two slices of plain bread into the pan, butter side down.
Evenly distribute 1/4 of the drained peppers, spinach, and cooked mushrooms over each slice of bread. Cover the sandwich with the other slice of bread that has the gigande spread on it. After about 2-3 minutes, flip the sandwiches to brown the other side for about 2-3 minutes more. Repeat for the next set of sandwiches. These sandwiches can also be made in a George Foreman grill or a panini maker.