Cut a 1cm edge around the tofu blocks and scoop out the inside with a spoon. Chop the removed tofu finely.

In a bowl, mix natto, its sauce, mustard, and egg for about 2 minutes until fluffy. Add the chopped tofu and mix well.

Place the tofu shells on a baking tray lined with aluminum foil. Fill them with the natto mixture and top with shredded pizza cheese. Bake in a toaster oven for about 10 minutes until the cheese is golden brown.