Prepare your vegetables by washing and slicing them as needed.
In a skillet, melt 1 tablespoon of butter over medium heat.
Add the sliced zucchini and bell pepper to the skillet and sauté for about 4-5 minutes.
Incorporate the fresh spinach and sprinkle with garlic powder, salt, and pepper; cook for another 1-2 minutes.
Remove the vegetables from the skillet and set aside, keeping them warm.
In the same skillet, add another tablespoon of butter and place two slices of bread in the pan.
Once the bread is lightly toasted on one side, flip it over and layer with mozzarella cheese, followed by the vegetable mixture, feta, and the top slice of bread.
Cook for another 3-4 minutes on medium-low heat, carefully flipping the sandwich halfway through.
Remove the sandwich from the skillet and let it rest for a minute before slicing.