In a large saucepan cook onion in hot oil until tender. Sprinkle flour over onion and stir to coat. Add the asparagus, milk, chicken broth, potatoes, salt, and red pepper.
Cook and stir until thickened and bubbly; reduce heat. Simmer, covered, for 10 to 12 minutes or until vegetables are just tender, stirring occasionally. Add cheddar cheese, tomato, and sour cream; stir until cheese melts. Makes 4 main-dish servings.