Thaw the frozen spinach in a saucepan at a low temperature with a little water (otherwise it will burn), then pour it into a lightly greased casserole dish. Place the cod on top of the spinach and brush with the red pesto. Cut the tomatoes and mozzarella into slices. First put the tomatoes on the cod, then the mozzarella.
Then bake the whole thing in a hot oven at 180 ° C for about 20 minutes.