In large saucepan, heat oil over medium heat. Add onions and garlic, sauté until softened (about 3-5 minutes). Add broccoli; sauté 2-3 minutes.
Add broth; bring to a boil, stirring occasionally. Reduce heat; simmer until broccoli is heated through (about 10 minutes), stirring occasionally. Add cheeses and milk; cook until melted; stirring frequently.
Meanwhile, in small bowl, combine corn starch and water until dissolved. Add to soup; simmer until soup thickens (about 5 minutes); stirring frequently. Serve with croutons, if desired.