Heat oil and butter in a Dutch oven over medium-high heat until butter melts. Add leeks and garlic; sauté 2 to 3 minutes or until leeks are tender.
Reduce heat to medium. Add flour; cook 1 minute. Gradually add broth, stirring constantly until slightly thick. Add potato and chervil. Bring to a boil; cover and simmer 30 to 35 minutes or until potato is tender.
Place 1/2 of soup in a food processor; process until smooth. Return mixture to Dutch oven; stir well. Stir in milk, cheese, salt, and pepper; cook 2 minutes or until thoroughly heated. Top with green onions, if desired.