Make the sauce: In a bowl, whisk together soy sauce, honey, broth, garlic, ginger, vinegar, sesame oil, black pepper, and red pepper flakes if using.
Add to cooker: Place the beef in your slow cooker or Instant Pot. Pour the sauce over the top and stir to coat.
Cook it: Slow cooker: Cook on Low for 6–7 hours or High for 3–4 hours, until the beef is fork-tender.
Instant Pot: Seal and cook on High Pressure for 35 minutes.
Natural release for 10 minutes, then quick release.
Stovetop: Add beef and sauce to a Dutch oven. Bring to a gentle simmer, cover, and cook on low for 1.5–2 hours, stirring occasionally, until tender.
Thicken the sauce: Stir the cornstarch slurry. Switch cooker to High or set the Instant Pot to Sauté.
Stir in slurry and cook 3–5 minutes until the sauce is glossy and lightly thickened. On the stovetop, simmer after adding slurry until it coats a spoon.
Taste and adjust: Add a splash more soy for saltiness, honey for sweetness, or vinegar for brightness as needed.
Serve: Spoon over rice or noodles. Garnish with green onions and sesame seeds.
Enjoy.