Prepare the bread: Brush one side of each bread slice with olive oil or spread with butter. This will be the outside of the sandwich for a perfectly golden, crispy finish.
Assemble the sandwich: On the unbuttered side of two bread slices, layer the mozzarella or provolone cheese, followed by crumbled feta, sun-dried tomatoes, olives, and fresh spinach. Sprinkle with oregano, salt, and pepper to taste. Top with the remaining bread slices, buttered side facing out.
Grill the sandwich: Heat a skillet or griddle over medium heat. Place the sandwiches in the skillet and cook for 3-4 minutes on each side, pressing gently with a spatula, until the bread is golden brown and the cheese is melted.
Serve and enjoy: Let the sandwiches cool slightly before slicing in half. Serve with a side of hummus, tzatziki, or a fresh cucumber salad.