Bring a good 4 litres of salted water to the boil (approx. 1 teaspoon salt per litre of water). Peel and finely chop the onion. Halve the ham and then cut into strips.
Cook the pasta in boiling salted water according to the instructions on the packet. Melt 2 tablespoons of fat. Sauté onion in it until transparent. Dust with flour and sauté until light brown. Add 350 ml water and milk while stirring.
Simmer for about 5 minutes while stirring. Season to taste with salt, pepper and stock. Stir in cheese and melt in it.
Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Drain the pasta. Mix with the ham and the frozen peas into the sauce. Pour into an ovenproof dish. Crumble toast over it and spread 3 tablespoons of fat in flakes on top.
Bake in a hot oven for about 25 minutes.
Clean the salad, wash and spin dry. Pluck into small pieces. Mix sour cream and lemon juice and season with sugar. Mix with the salad. Serve pasta casserole with salad.