Thaw the fish and cut roughly. Peel and dice the potatoes. Wash tomatoes, then quarter. Clean the spring onions and cut into small rings.
Brush a baking dish with the tablespoon of oil. Put the above ingredients in the baking dish. Pour the pesto over it. Pepper and salt, then mix everything well, preferably by hand.
Place the casserole dish in the oven at 180 ° C for 30 - 35 minutes and serve immediately.