Steam scallops and mussels until just cooked, and then cool to room temperature. Combine the lemon zest and juice, garlic, oil, sugar and dill in a bowl. Gently toss in the seafood, bell pepper, tomatoes, red onion, salt and pepper. Cover and marinate in the fridge 30 minutes. Divide and arrange salad greens on four chilled plates. Artfully arrange seafood mixture in the centre and serve.