Preheat oven to 400°F.
For the sweet potato salad: Peel and cut potatoes into large pieces. Toss in a bowl with olive oil, 1/2 teaspoon salt and 1/4 teaspoon pepper. Bake on large baking sheet for 18-20 minutes or until just tender. Meanwhile, hard boil the eggs.
Place potatoes back into a large bowl and drizzle with vinegar. Allow to cool completely. Chop eggs, green onion and apple. Add to bowl along with mayo and yogurt; blend well. Refrigerate until ready to serve.
Preheat grill. Place steak in a glass container. In a separate dish, mix together the remaining 1 1/2 teaspoon salt and 1/4 teaspoon pepper, paprika, brown sugar and chili powder. Press seasonings into steak and allow to rest for 20 minutes at room temperature.
Place steak on grill and cook for 8-10 minutes per side or until desired doneness.
Top steak with chimichurri sauce (if desired) and serve with potato salad and watermelon on the side.