Cook the pasta in enough boiling salted water for 10-12 minutes, adding the peas in the last 2 minutes.
Drain both in a colander. Peel and finely dice the onions.
Set aside some diced onions and peas. Mix tomatoes, onions, peas and pasta in a bowl.
Mix egg yolks with mustard, pepper, salt and juice of one lemon, add oil while stirring drop by drop at the beginning, then in a thin stream.
Drain tuna, add to mayonnaise and puree everything.
Stir whipped cream and milk into the sauce, as well as the capers. Season with salt and pepper and mix into the salad.
Serve garnished with diced onions and peas.