Preheat your oven to 350°F (175°C) and place the oven rack in the center position.

Carefully slice each croissant horizontally, creating a pocket for fillings.

Generously spread pesto on the bottom half of each croissant.

Place slices of fresh mozzarella on top of the pesto, overlapping slightly to cover the surface.

If using, sprinkle chopped sun-dried tomatoes on top of the mozzarella.

If using, sprinkle a tiny pinch of red pepper flakes on top of the mozzarella.

Gently fold the top half of the croissant back over the filling.

Place the filled croissants on a baking sheet and bake for 10-12 minutes, until cheese is melted and bubbly and croissants are lightly golden brown.

Let cool for a minute or two before serving.