Peel potatoes and cut into ½-inch cubes; wash and cut baby marrows, new carrots and green beans into ½-inch lengths. Boil each vegetable until tender but still quite firm. Do not overcook.
Seed and cut large fresh tomatoes into ½-inch cubes. Arrange vegetables in colourful groups on a large shallow serving dish; sprinkle with salt and freshly ground black pepper, to taste. Serve with Aï