Spread the olive oil butter (or regular softened butter) over 1 side of each of the slices of bread. (*1) Set aside

In a skillet, heat 2 tablespoons of olive oil on medium high heat. Add the mushrooms and cook stirring frequently for about 3-5 minutes or until almost fully cooked. 

Once the mushrooms are almost fully cooked, add the spinach, sprinkle everything with salt and pepper, and saute until the spinach is wilted (about 2 minutes). Remove from heat. 

Heat a dry skillet or griddle over medium heat. 

Assemble the grilled cheese sandwich by placing the butter side of the first slice face down, then add 1 slice of swiss, a bit of the mushroom spinach mixture, another slice of swiss, and then top with the 2nd slice of bread with the buttered side facing up. 

Place the sandwich on the skillet or griddle and cook until the cheese starts to melt and the bottom is lightly browned. Flip and do the same with the other side. 

Remove from heat and serve.