Heat 2 tablespoons of oil over low heat in a soup kettle. Prepare onions, carrots, celery, and asparagus and measure peas.
When ready to sauté, increase heat to medium-high and add onion, carrots, and celery; sauté until softened, about 5 minutes. Stir in tarragon and then couscous, followed by broth; cover and bring to a simmer. Reduce heat to medium-low and continue to simmer until most of the liquid has absorbed but dish is still moist, about 10 minutes.
While couscous cooks, place asparagus in a (12-inch) skillet with remaining 1 tablespoon of oil, 1/4 cup of water, and a sprinkling a salt and pepper. Cover skillet, turn burner to high, and heat until vegetables start to steam. With skillet still covered, cook until all of the liquid has evaporated and vegetables start to sauté, about 5 minutes. Remove lid, add peas, and continue to cook until peas thaw, a few minutes longer.
Stir in vegetables, spinach, and optional cheese into the couscous and cook to wilt spinach and blend flavors, a couple of minutes longer. Adjust seasonings, including salt and pepper to taste. Serve with an optional sprinkling of cheese.