In a small bowl, mix the softened butter with minced garlic, oregano, and a pinch of salt and pepper to create a garlic butter spread.
Heat a grill pan or a non-stick skillet over medium heat.
Spread the garlic butter mixture on one side of each slice of whole-grain bread.
On the unbuttered side of two slices, layer half of the mozzarella cheese evenly.
Place the sliced zucchini over the cheese, then sprinkle with crumbled feta cheese.
Add the remaining mozzarella on top of the zucchini and then close the sandwich with the other two slices of bread, buttered side out.
Grill the sandwiches for about 3-4 minutes on each side, or until the bread is golden brown and the cheese has melted, pressing down slightly with a spatula.
Once ready, remove from heat and let cool for a minute. Cut the sandwiches in half diagonally.
Garnish with fresh basil leaves on the side and serve immediately.