Place steak, oregano, onion powder, sugar, and 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt in a medium bowl; season generously with pepper and toss to coat. Let sit at least 10 minutes or cover and chill up to 8 hours (no need to bring to room temperature before cooking if chilling).

Heat 2 Tbsp. oil in a large high-sided skillet or medium Dutch oven with a tight-fitting lid over medium. Add scallions, chopped dill, half of garlic, and a large pinch of salt; cook, stirring occasionally, until scallions are softened, about 5 minutes. Add spinach in 2–3 handfuls, stirring and letting wilt slightly after each addition before adding more, 5–7 minutes total. Pour in 1 cup water and bring to a boil; season with more salt if needed. Add couscous and stir to break up any lumps. Remove from heat; quickly cover with lid and let sit 10 minutes.

Meanwhile, heat 1 Tbsp. oil in a large skillet, preferably cast iron, over high. Arrange steak in a single layer; cook, undisturbed, until deeply browned underneath, 6–8 minutes. Reduce heat to medium; continue to cook, stirring often, until meat is just cooked through, about 1 minute more. Remove from heat. Add remaining garlic and a large pinch of salt; toss to combine. (Residual heat from pan will cook out any raw garlic flavor.)

Uncover couscous and fluff with a fork to separate grains. Spoon into a large shallow bowl or onto a platter. Top with steak. Cut lemon into quarters and squeeze juice from 1 quarter over steak. Scatter cucumber and some dill sprigs on top; drizzle with oil.

Combine yogurt and a generous pinch of salt in a small bowl; squeeze juice from 2 lemon quarters into yogurt. Stir to combine. If sauce is thick, stir in water, a spoonful at a time, to loosen.

Serve steak and couscous with yogurt sauce and remaining lemon quarter and more lemon wedges for squeezing over.