Begin by washing fresh spinach thoroughly under cold water, then sauté in a skillet over medium heat for 2-3 minutes until wilted.

Spread butter on one side of each slice of bread. On the unbuttered side of one slice, layer sautéed spinach and crumbled feta, adding any optional ingredients.

Preheat a skillet over medium heat and place the sandwich in it, cooking for about 3-4 minutes until the bottom is golden brown.

Flip the sandwich carefully and cook for another 3-4 minutes until the cheese is melted and the bread is toasted.

Let it rest for a minute, then slice diagonally and serve warm, preferably with tomato soup or salad.