Toast the croissant if desired then carefully cut it in half.
Cut the tomato into thin slices and the mozzarella cheese into slices bout 1 cm thick.
Clean some anchovy fillets then let them marinate in olive oil. In order: place the tomato slices, mozzarella cheese, and then anchovy fillets on the croissant.
If you’re not a fan of anchovies, add balsamic vinegar, garlic, and oregano to the marinade (or leave off the anchovies. We won’t judge)