Preheat oven to 400°. Snap off and discard tough ends of asparagus. Coat asparagus lightly with cooking spray, and arrange in an aluminum foil-lined jelly-roll pan. Sprinkle with 1/2 tsp. kosher salt and 1/2 tsp. pepper.
Bake at 400° for 16 minutes or until tender and slightly browned, turning after 8 minutes. Remove from oven, and let cool.
Meanwhile, melt 2 Tbsp. butter in a large Dutch oven over medium-high heat; add onion, and cook, stirring frequently, 3 minutes or until tender. Add remaining butter, and stir until melted. Add potatoes, and cook, stirring occasionally, 3 minutes. Stir in chicken broth. Bring to boil; reduce heat to medium, and simmer 6 minutes or until potatoes are tender.
Chop asparagus into 1/4-inch pieces. Add chopped asparagus to soup; cook, stirring occasionally, 4 minutes. Remove from heat, and let cool 10 minutes.
Process asparagus mixture, in batches, in a blender or food processor 8 to 10 seconds or until smooth, stopping to scrape down sides. Return asparagus mixture to Dutch oven. Cook over medium heat, stirring occasionally, 5 minutes or until thoroughly heated. Season with salt and pepper to taste. Serve immediately.