Preheat oven to 350 degrees F (175 degrees C). Place acorn squash cut-side down on baking sheet.
Bake squash in the preheated oven until tender when pierced with a fork, about 45 minutes. Flip to face up and let cool slightly.
Bring 2 1/4 cups water to a boil in a saucepan; remove from heat and stir in couscous and cilantro. Cover saucepan and let stand until water is absorbed completely, about 10 minutes. Fluff couscous with a fork.
Bring a large pot of water to a boil. Add spinach and cook until wilted, 3 to 5 minutes. Drain; use paper towels to squeeze out excess moisture. Fold into couscous.
Melt margarine in a skillet. Add onion; cook and stir until browned, about 10 minutes. Mix onion into the couscous mixture. Stir in seasoned salt, salt, and pepper.
Stuff acorn squash halves with couscous mixture. Sprinkle feta cheese on top.
Bake in the preheated oven until heated through, about 15 minutes.