Add 0.3 cups dry cooking wine to a small saucepan over medium heat. Bring to a simmer and cook for 2 minutes to cook off the raw alcohol.

Add 0.5 cups soy sauce (preferably koikuchi), 2 tablespoons usukuchi (light soy sauce), 0.3 cups mirin, 1 teaspoon white sugar, 1 teaspoon minced garlic, 0.3 teaspoons ginger powder, and 0.5 teaspoons onion powder. Stir to combine and bring to a gentle simmer over medium-low heat. Do not let it boil hard.

Reduce heat to the lowest setting and simmer gently for 15 minutes, stirring occasionally. It should reduce slightly and smell deeply savory.

Remove from heat and stir in 0.3 cups katsuobushi (bonito flakes). Let steep off-heat for 10 minutes -- this is your main umami source in this version, so don't skip it.

Strain through a fine-mesh sieve, pressing on the solids to get every drop. Stir in 1 teaspoon sesame oil. Taste -- intensely salty and savory is correct. It's a concentrate.

Cool completely, then jar and refrigerate. Use 2-3 tablespoons per bowl of tonkotsu broth. Start low and adjust to taste.