Roughly mash the cooked broccoli, then mix with the grated cheese, tuna, greek yogurt, beaten egg and season with black pepper.
Take a croissant and slightly open the whole you have made. Spoon an eighth of the mixture into it, place on a baking tray, and then repeat with the rest of the croissants.
Place in the oven for 12-15 minutes, until the croissants are golden brown and the filling has melted.