Pre-heat a lightly oiled barbecue or chargrill pan over medium high heat. Combine garlic, soy, sesame oil and mirin in a small bowl. Rub marinade over lamb and season.
Cook lamb for 12 to 15 mins on each side, brushing with marinade, or until cooked to your liking. Set aside to rest for 10 minutes and thinly slice.
Meanwhile, preheat oven to 200°C (180° fan-forced). In a large roasting pan lined with baking paper, place the pumpkin, potatoes, cauliflower and broccoli. Drizzle with the olive oil, and sprinkle with the spices. Season and toss to coat. Roast vegetable for 30 to 35 minutes, turning once, or until golden and tender. Add spinach to the pan, toss and sprinkle with hazelnuts.