Croatian White Onion Sauce

Bring onion and water to a boil in a saucepan. Cover reduce heat and simmer until tender 15 to 20 minutes. Transfer onion and water to a blender; puree until smooth.

Heat oil in a saucepan over medium heat. Whisk in flour to form a thick paste the consistency of cake frosting. Continue cooking whisking constantly until flour has turned golden and begins to smell like toast about 20 minutes.

Stir onion puree into flour paste. Cook and stir until thickened to sauce consistency adding more water if necessary about 2 minutes. Stir in wine vinegar and salt.