How to Make It
Melt butter in a small saucepan over medium heat. Add onion; cook 4 minutes or until tender, stirring frequently. Add fennel and garlic; cook 1 minute, stirring constantly. Stir in wine; cook 1 minute. Add stock and thyme; bring to a boil. Reduce heat; simmer 10 minutes. Strain mixture through a sieve into a bowl; discard solids.
Return reserved liquid to pan. Gradually add flour and salt, stirring with a whisk. Bring to a boil; cook 1 minute or until slightly thick, stirring constantly with a whisk.