In a large bowl, combine dressing ingredients; set aside. Boil sweet and baby potatoes until tender, 8 to 10 minutes.
Meanwhile, place steaks on a greased grill over medium-high heat; close lid and grill, turning once, until cooked to your liking (6 to 8 minutes for medium-rare). Let stand for 5 minutes on a cutting board; thinly slice against the grain. Reduce grill heat to medium; thread tomatoes onto skewers and grill, turning once, until lightly charred, a couple of minutes.
To the salad dressing bowl, add drained warm potatoes, yellow pepper, celery, onion and spinach, stirring gently to coat and wilt spinach leaves. Divide salad, steak slices and tomato skewers among 4 plates.
Per serving: 344 calories, 30 g protein, 9 g fat (3 g saturated fat), 37 g carbohydrates, 5 g fibre, 55 mg cholesterol, 115 mg sodium