Defrost and drain the spinach. Prepare pasta in boiling salted water according to package instructions. Peel garlic and press it through a garlic press. Wash the fish, dab dry, remove any bones and cut flat into slices.
Whisk the sour cream, milk, garlic, herb mixture and eggs, sieve the starch over it and season with salt, nutmeg and pepper. Drain the pasta, rinse with cold water and drain well. Layer about 1/3 of the pasta in a greased casserole dish (approx. 1.5 litres capacity).
Lay salmon and spinach alternately in the middle. Add the remaining noodles to the edge. Pour the sour cream and egg milk over everything. Mix both cheeses with breadcrumbs and sprinkle over the casserole.
Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 45-60 minutes until golden brown and the egg milk has set. Cover with aluminium foil if necessary.