Start by gathering two or four thick slices of sourdough bread, ensuring they are fresh for optimal taste and texture.

Generously spread softened butter on one side of each slice, evenly for that golden crisp. Place the butter-side down in a skillet over medium-low heat.

On the unbuttered side of each slice, layer 3-4 slices of mozzarella cheese. Add a heaping tablespoon of basil pesto and sprinkle optional sun-dried tomatoes. Layer in crispy bacon if desired, then top with more mozzarella.

Carefully place another slice of sourdough on top of each stack, pressing down gently.

Adjust the skillet heat to medium-low if necessary, ensuring it's evenly heated before cooking.

Fry the sandwiches for about 3-4 minutes on each side until golden brown. Flip when the underside is golden and the cheese starts melting.

Once browned, remove from skillet and let them cool briefly before cutting in half. Serve immediately with tomato soup or salad.