Peel and wash the potatoes and cut them into thick sticks. Put potatoes in boiling salted water and precook for about 4 minutes. Pour onto a sieve and rinse under cold water. Drain potatoes and let them cool down.
Clean, wash and drain the peppers and courgettes. Cut the vegetables into pieces. Clean oyster mushrooms and, depending on size, leave whole or cut into pieces. Arrange potatoes, vegetables and mushrooms, cover and keep cool until serving.