In a bowl, whisk together white miso, umeboshi paste, mirin, sake, sugar, yuzu juice, soy sauce and oil until smooth. Taste and adjust: add more ume paste for salt/tartness, more sugar for sweetness, or more citrus for brightness.

Portion and reserve

Reserve about 2 tablespoons of the marinade for basting/finishing. Place remaining marinade in a shallow dish large enough to hold the fillets.

For the Pickled Red Onion

Quick-pickle onions

In a small saucepan combine rice vinegar, water, sugar, salt and optional peppercorns. Bring to a gentle simmer to dissolve sugar and salt, then remove from heat. Place sliced red onion in a jar or bowl and pour the hot pickling liquid over the onions. Let cool to room temperature, then cover and refrigerate at least 30 minutes (preferably 1 hour) to mellow. These can be made a day ahead.

For the Black Cod Kushiyaki

Prepare fish

Pat black cod fillets dry with paper towel. Lightly season the skin side with 1/2 tsp kosher salt divided among fillets. If fillets are large, portion into 3–4 oz pieces. Thread each piece onto two parallel soaked bamboo skewers (kushiyaki style) so the fish stays flat and is easy to turn.

Marinate

Brush both sides of each skewered fillet with the ume miso marinade (use the portion in the shallow dish). Cover and refrigerate for 30–60 minutes. Do not exceed 2 hours to avoid over-salting or breaking down the fish too much.

Preheat grill or broiler

If using a grill: preheat to medium-high (about 400°F / 200°C). If using broiler: position rack 6 inches from heat and preheat broiler. Lightly oil grill grates or a broiler pan.

Grill / broil the kushiyaki

Brush the fish lightly with oil. Place skewers on the grill or under the broiler, skin side down first. Cook 3–4 minutes until the marinade firms and the skin begins to crisp and brown. Carefully flip and cook 2–3 more minutes until the fish is opaque and flakes easily. Total cook time will be ~5–7 minutes depending on thickness. Avoid overcooking.

Finish with reserved marinade

During the last 30–60 seconds of cooking, brush the top of each fillet with the reserved 2 tablespoons of ume miso for glossy finish and an extra hit of flavor. If desired, flash-char with a kitchen blowtorch for 10–15 seconds to caramelize the glaze (optional).

For Plating & Garnish

Transfer kushiyaki to a serving platter. Top each skewer with a small mound of pickled red onion. Sprinkle with chopped chives (or mitsuba) and toasted sesame seeds. Serve with lemon or yuzu wedges on the side for squeezing. Offer any leftover pickling liquid or extra marinade warmed slightly as a dipping sauce if desired.

Serve immediately while fish is warm. Leftover pickled onions keep refrigerated up to 1 week; cooked fish is best the day it’s made.