Mix the sauce of record: Whisk the gochujang, soy sauce, honey, garlic, ginger, rice vinegar, sesame oil, and broth in a bowl until smooth. Taste it raw it should be sweet, punchy, and pleasantly aggressive. The oven will soften the edges, so trust the first read.
Load the dish, frozen: Arrange the frozen dumplings in the 9×13 in a single snug layer, pleats up if they have them, with the spinach tucked between and around them if you’re using it. Pour the sauce evenly over the whole scene. Here’s what the frozen state is buying you: rigid wrappers hold their shape as the sauce steams around them, and they release almost none of their own water the exact trait that stops this dish from turning into sauce-soup. Thawed dumplings weep, sag, and cross-link into a doughy slab. If a bag ever leaves the freezer before this step, the recipe changes personality.
The steam pass: Cover the dish tightly with foil and bake at 375°F for 25 minutes. The covered phase steams the dumplings through to the very center of the pleats while the sauce bubbles and thickens around them. By the end, the kitchen should smell like a very friendly Korean barbecue and the sauce should be actively burbling at the edges.
The cheese pass: Uncover and scatter the mozzarella and parmesan over the top. Bake uncovered for 8–10 minutes, until the cheese is fully molten and the edges have started to brown in patches. If you want leopard spots, hit it with the broiler for 1–2 minutes pro tip: mozzarella goes from blistered to ruined in about 45 seconds, so stand there and watch it like a hawk with a soft spot.
Rest and finish: Let the dish sit 5–8 minutes before serving long enough for the sauce to settle and the cheese to set just shy of solid. Shower over the scallions and sesame seeds, add a drizzle of chili crisp if you like the heat to notice you, and serve with spoons, because this is a scoop-then-eat situation.
