Place the lamb chumps on a baking tray. Drizzle the olive oil, lemon zest and juice, then sprinkle over the garlic and oregano. Season with salt and pepper and set aside to marinate.
Meanwhile, heat a large pan over a high heat. Drizzle with oil and fry off the vegetables one at the time until they are all browned and beginning to soften. After all the vegetables have been cooked, gently sauté the garlic for about 1 minute. Place all the vegetables and the garlic in a sauté pan, pour in the tomato passata and allow the vegetables to simmer for 10 to 15 minutes over a medium to low heat.
When the coals are lightly coated with ash and are a medium heat, oil the grill grates. Grill the lamb on the barbecue for 8 to 10 minutes on both sides, until pink in the centre. Remove the lamb from the barbecue and rest for at least 5 minutes before slicing.
Spoon the ratatouille onto four plates and top with the sliced lamb. Serve with baby potatoes and some rocket.