Heat a skillet over medium heat and add olive oil. Sauté chopped spinach for 2-3 minutes until wilted. Remove from heat and mix with crumbled feta.

In a bowl, whisk the eggs with salt and pepper. Return the skillet to heat, pour in the eggs, and cook over medium-low heat. Add the spinach and feta mixture and stir until eggs are fully cooked, about 3-4 minutes.

Assemble the sandwich by placing the spinach and feta egg mixture on one slice of bread, add any optional ingredients, and top with the second slice.

Toast the sandwich in the skillet or toaster until golden brown and crispy, about 2-3 minutes on each side.