PREHEAT oven to 450F. Set a rack on a baking sheet.
COMBINE green onions with 1/4 cup mint, parsley and garlic in a large bowl. Stir in lamb, tomato paste, chili flakes and salt just until combined. Season with pepper.
SPREAD lamb mixture over pizza crust, right to the edges, until even. Lay pizza on rack.
BAKE in centre of oven until lamb is cooked through, about 10 min.
STIR lemon juice with tahini in a small bowl. Stir in 2 tbsp water. Season with pepper.
COMBINE arugula with tomatoes, red onion and remaining mint in a medium bowl. Transfer hot pizza to a cutting board and let stand 5 min. Top with arugula-tomato mixture, drizzle with tahini dressing, then cut into 8 wedges.