Heat 1/4 cup of the oil in a large saucepan over medium heat until it shimmers. Then whisk in 1/2 cup of the flour. Cook, stirring constantly, for 3 to 5 minutes until the mixture darkens slightly and smells toasty.

Slowly and steadily add 4 1/4 cups of the grape juice, whisking the whole time so no lumps form. Keep whisking until the mixture becomes smooth.

Bring the mixture to a boil, reduce the heat to a simmer, and cook for about 1 hour, whisking often, until it has reduced by about half and is very thick but still pourable.

Thread a needle with doubled thread and knot the bottom end. String walnut halves tightly without gaps. When tatara is cool, dunk walnuts fully into the tatara and let the excess drip before hanging to dry.

Let the strings dry for 24 hours. Make a second batch of tatara and dip each string again, then hang for another 48 to 72 hours.

Slide the string out and slice the candy into rounds. Store at room temperature wrapped in a towel or paper for up to 3 months.