Soak oats overnight in about 1/4 litre of cold water. Pour oats onto a sieve, drain and cook in 1/2 litre of boiling water for one hour. Pour onto a sieve and drain again.
Onions peel and cut into cubes. Heat two tablespoons of oil in a saucepan, fry the onions in it until transparent, add the oats and fry for ten minutes. Mix eggs and crème frâiche, season well with salt and pepper.
Peel and wash the potatoes and grate them into the egg mixture. Clean, wash and cut the leek into rings. Celery clean, wash and cut into cubes. Peel, wash and slice carrots.
Grease the pie dish (28 cm Ø), pour in half of the potato and egg mixture, layer the oats and vegetables on top. Layer the remaining potato and egg mixture. Bake in the preheated oven (electric oven: 200°C/ gas: level 3) for 50 to 60 minutes.
In the meantime, clean, peel and roughly grate the beetroot. Wash the apple, quarter it, core it and grate it roughly. Mix beetroot and apple. Squeeze lemon, mix lemon juice and remaining oil well.
Season with salt, pepper and sugar and pour over the beetroot. Wash the lamb's lettuce thoroughly. Cut the pie into pieces and arrange on a plate with the beetroot salad and the lamb's lettuce. Garnish the pie with parsley and chives.