Melt butter in a medium saucepan over medium-high heat.
Add onion to pan; saut 5 minutes or until onion is tender.
Sprinkle with flour; cook 1 minute, stirring onion mixture constantly.
Add potato, milk, broth, and 1/2 cup water to pan; bring to a boil. Cover, reduce heat, and simmer 10 minutes.
Add 1/2 cup reduced-fat sharp cheddar cheese and ground red pepper; cook 2 minutes or until cheese melts, stirring frequently. Top each serving evenly with 1 1/2 teaspoons chopped green onions.