Peel and wash the potatoes and carrots. Peel onions and garlic. Crush garlic, cut onions into rings. Wash and chop the thyme. Mix cream, milk, garlic and thyme. Season with salt and pepper
Wash the fillets if necessary, dab dry. Season with salt and pepper. Fry them in hot oil, remove them. Fry the onions in the frying fat until golden brown. Dust with flour, sweat and deglaze with the cream mixture. Bring to the boil, simmer for about 3 minutes
Slice or cut the potatoes and carrots into thin slices. Arrange with the fillets in a large greased casserole dish. Pour the onion cream over it. Bake in a hot oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for about 1 1/4 hours