Peel and wash the potatoes. Cover and cook in boiling salted water for about 20 minutes. Covered frozen peas and cook in 100 ml hot water for about 10 minutes until soft. Drain in a sieve, pour into a small bowl and mash with a fork.
Drain the potatoes. Mash them with a potato masher, adding milk and butter in pieces. Stir in the pea purée. Season to taste with salt and pepper.