Preheat oven to 220°C/200°C fan-forced. Place potato, sweet potato, garlic and oil in a bowl. Toss well to combine. Place mixture, in a single layer, onto a large oven tray. Season with salt and pepper. Roast for 20 minutes.
Turn potato and sweet potato. Add capsicum and red onion tray. Roast for 15 minutes then add pine nuts to vegetables to roast for a further 5 minutes or until vegetables are browned and tender. Set aside for 10 minutes to cool slightly.
Meanwhile make dressing. Place lemon juice, oil, mustard and oregano in a screw-top jar. Secure lid. Shake well to combine.
Place rocket, pine nuts and vegetables in a large bowl. Pour over dressing. Toss gently to combine. Serve.