In a large bowl, place the lamb chunks and add all the marinade : red onion, garlic, lemon zest and juice, oregano, Dijon mustard, olive oil, salt, and pepper.
Mix thoroughly to ensure the lamb is evenly coated.
Cover and refrigerate for at least 2 hours, or ideally up to 12 hours for maximum flavor.
Preheat your oven to 300°F / 150°C / gas mark 2.
Transfer the marinated lamb and all the contents of the bowl into a large, heavy-bottomed casserole dish or Dutch oven.
Add the rutabaga, green peppers, and tomato.
Stir everything together to combine.
Cover the dish tightly with a lid or a double layer of aluminum foil to seal in the steam.
Place in the preheated oven and bake for 3 hours and 40 minutes.
Carefully remove the lid or foil. If using, stir in the diced halloumi. Return the dish to the oven, uncovered, and bake for another 20 minutes, or until the lamb is perfectly tender.
Remove from the oven and let the Kleftiko rest for 10 to 15 minutes before serving.
Garnish generously with fresh herbs and serve immediately.
Don't be afraid to swap vegetables based on what's in season. Chunks of potato, carrot, or fennel would also be delicious additions.