1. Preheat grill to 350° to 400° (medium-high). Rub steak with 1 Tbsp. oil, 1 1/2 tsp. kosher salt, and 3/4 tsp. pepper.
2. Brush squash and onion with remaining 2 Tbsp. oil; sprinkle with remaining 1/2 tsp. kosher salt and 1/4 tsp. pepper.
3. Grill steak and vegetables, covered with grill lid, over 350° to 400° (medium-high) heat 5 to 7 minutes on each side or until steak reaches desired degree of doneness and vegetables are tender. Remove steak and vegetables from grill; squeeze juice from lemon over steak and vegetables. Cover steak and vegetables with aluminum foil, and let stand 10 minutes.
4. Meanwhile, prepare couscous according to package directions.
5. Cut steak across the grain into thin slices. Cover and chill half of sliced steak (about 1 lb.) up to 2 days. Top couscous with vegetables; sprinkle with feta cheese. Serve with remaining half of steak and Chunky Cucumber-Mint Sauce.
Chunky Cucumber-Mint Sauce: Stir together 1 cup plain yogurt; 3 Tbsp. sour cream; 1 small peeled, seeded, and chopped cucumber; 4 tsp. chopped fresh mint; and salt and pepper to taste. Makes about 1 3/4 cups; Prep: 10 min.