In a large saucepan, cook and stir onion in olive oil over medium heat until tender. Add broth and potatoes; bring to a boil. Reduce heat; cover and simmer for 10-15 minutes or until potatoes are fork tender.
Stir in the corn, 1/2 cup milk, salt and pepper. In a small bowl, whisk flour and remaining milk until smooth. Stir into soup; return to a boil. Cook and stir for 2-3 minutes or until thickened. Sprinkle with parsley if desired.