Preheat the oven to 180°C, fan 160°C, gas 4. Put the tomatoes on a baking tray and top with half a tablespoon of the oil and the oregano. Roast for 15 minutes.
Meanwhile, heat 1 tablespoon of the remaining oil in a pan and cook the onion for 5 minutes, adding the garlic for the last minute. Stir in the butter beans, stock, crème fraîche and pesto. Simmer for 5 minutes.
Brush the lamb steaks with the remaining oil and season with black pepper. Heat a frying pan over a high heat, add the steaks and cook for 3-4 minutes on each side. Remove the steaks from the pan and allow to rest. Add the spring greens to the pan and stir-fry for 5 minutes.
Serve the lamb steaks on top of the pesto butter beans and spring greens, with the roasted cherry tomatoes on top.