Peel and grate the Asian pear and onion; mince the garlic and ginger. Thinly slice the scallions.
In a medium saucepan, whisk together the soy sauce, water, light brown sugar, honey, gochujang, rice vinegar, grated pear and onion (with their juices), garlic, ginger, and black pepper.
Bring to a gentle simmer over medium heat, stirring often, until the sauce is glossy and lightly thickened enough to coat the back of a spoon, 6–10 minutes.
Remove from the heat and stir in the toasted sesame oil and sliced scallions.
Let cool 10–15 minutes. Use as a marinade (coat meat and refrigerate: beef or pork 2–12 hours, chicken 2–8 hours), as a basting glaze (brush on during the last 5 minutes of grilling), or as a dipping sauce. Store refrigerated up to 1 week.