Zest and juice lemon. Reserve zest, chilled, for couscous. Combine 3 tbsp. lemon juice (save extra for other uses) with the vegetable oil, garlic, soy sauce, and 1/2 tsp. each salt and pepper in a large resealable plastic bag. Add beef, seal bag, and chill at least 30 minutes and up to 24 hours.
Heat a grill to high (450° to 550°). Thread beef onto 8 soaked wooden skewers and discard marinade.
Prepare couscous according to package instructions. In a large bowl, combine couscous with reserved lemon zest, remaining 1/2 tsp. each salt and pepper, the olive oil, tomatoes, red onion, and parsley. Chill if you like or serve at room temperature.
Grill beef, turning once or twice, until grill marks appear and meat is done the way you like (cut to test), about 8 minutes total for medium-rare. Serve beef with couscous.