Melt the butter in a large skillet over low heat. Peel the onion and cut it into 4 even mini moons. Use 2 mini moons to dice into small squares. Add to the butter and cook over low heat for about 3 minutes or until onions are gold in color (be sure not to burn or brown the onions).

Wash the mushrooms and dice into medium squares. Season with garlic powder, paprika, black pepper, and salt to taste. Add to skillet, and stir to combine with onions. Increase the heat to about medium-low and cook for about 15 minutes to brown the mushrooms. Taste and adjust seasoning as necessary.

Combine the spinach with the mushrooms and sauté just until the spinach has wilted. Mix cheese in until fully melted (if using slices, I recommend ripping them up before combining them with this mixture).

Melt the 1 Tablespoon butter in the skillet and place two pieces of bread in the skillet and cook until bread is toasted until bread has gotten dark and crispy. Remove from heat, add mushroom mixture, slice in half, and serve!