Heat the grill to high. Bring a small pan of water to the boil and blanch the green beans and spring onions for 4 mins. Drain and dry well. Toss with 1 tsp oil, and transfer to a baking tray with the tomatoes, cut-side up. Season everything with black pepper, then grill for 10 mins until starting to soften and char. Leave to cool slightly.

Meanwhile, whisk the kefir, 1 tsp oil, mustard powder, vinegar and some seasoning together until smooth. Arrange the lettuce leaves, tuna, olives and eggs in shallow bowls or on plates. Pile on the onions, green beans and tomatoes, then drizzle over the dressing to serve.