In a bowl, combine soy sauce, mirin, brown sugar, sesame oil, minced garlic, minced ginger, black pepper, and gochujang (if using). Whisk until sugar dissolves and mixture is uniform.

Taste and adjust

Taste a small spoonful and adjust sweetness or salt: add up to 1 tbs more sugar for sweeter, or 1 tbs water to reduce saltiness.

Protein (choose one)

Prepare protein

If using beef, score or slice thinly across the grain (about 1/4" thick) for faster cooking. If using chicken thighs, trim excess fat and pat dry. If using tofu, press for 30 minutes and slice into 1/2" slabs.

Marinate

Place protein in a shallow dish or zip-top bag and pour 3/4 of the marinade over it, reserving 1/4 for basting/finishing. Massage to coat. Marinate in the refrigerator for at least 30 minutes (up to 4 hours for chicken or beef). For tofu, marinate 15–30 minutes.

Bring to room temperature

Remove protein from fridge 20 minutes before grilling to take the chill off; pat off excess marinade and reserve the excess for basting (do not reuse marinade that contacted raw meat unless boiled).

Vegetables & garnish

Prepare vegetables

Trim and halve scallions lengthwise. Cut onion into 1/2" thick rings. Slice zucchini into 1/4" rounds or lengthwise half-moons. Toss vegetables lightly with 1 tsp oil and a pinch of salt.

Grill vegetables

Grill vegetables alongside protein over medium-high heat until lightly charred and tender, about 2–4 minutes per side depending on thickness. Remove and keep warm.

Grilling & finishing

Preheat grill

Preheat grill or grill pan over medium-high heat until hot (about 10 minutes). Oil the grates lightly using a paper towel dipped in neutral oil and tongs.

Grill protein

Place protein on the hot grill. For thin-sliced beef: grill 1–2 minutes per side for medium-rare to medium, brushing once with reserved marinade and finishing with a quick brush of butter if desired. For chicken thighs: grill 6–8 minutes per side, flipping once, until internal temperature reaches 165°F (74°C), basting with reserved marinade during last 4 minutes. For tofu: grill 3–5 minutes per side until grill marks form, brushing with reserved marinade.

Finish and rest

Transfer cooked protein to a plate and let rest for 3–5 minutes. Top with a small pat of butter (optional) and sprinkle toasted sesame seeds and sliced scallions.

Serve

Serve grilled protein with grilled vegetables and lemon/lime wedges on the side. Offer steamed rice or lettuce leaves for wrapping if desired.

Safety note

If you reserved marinade that contacted raw meat, bring it to a rolling boil for at least 1 minute before using it as a sauce or discard it.