- 1 onion, peeled and quartered
- 3/4 cup water, or more as needed
- 1 teaspoon sunflower seed oil
- 1 teaspoon all-purpose flour
- 1 teaspoon white wine vinegar
- 1/4 teaspoon sea salt
- 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.