Toss the cabbage with the chopped green onions and the salt in a medium bowl. Let stand for 10 minutes or until cabbage is very wilted. Rinse and drain in a colander. Squeeze the cabbage and green onions, a handful at a time, to extract as much liquid as possible.
Place the cabbage and green onions in a mixing bowl. Add the ground pork, pepper, ginger, garlic, soy sauce, sake, and sesame oil. Add a tiny pinch of salt, but not too much because the cabbage has already been salted. Mix everything together gently, but thoroughly.
Fill a small bowl with water. Line a baking sheet with parchment paper and dust it with cornstarch.
Place 1-2 teaspoons of the pork and cabbage filling into the center of a gyoza wrapper. Dip your finger into the water and moisten the edges of the wrapper. Bring one edge of the wrapper up over the filling to meet the other edge. Press the edges together firmly.
Place the gyoza on the parchment pepper, plumping the bottom of the gyoza so that it stands with the pinched-together part facing up.
Repeat with remaining filling and wrappers. Cover and refrigerate until ready to cook (can be made up to 4 hours ahead).
Preheat the oven to 200 degrees. Heat 2 TBS. of the oil in a nonstick skillet over medium-high heat until the oil is very hot but not sizzling.
Place half the gyoza in the skillet, pinched part up, letting the gyoza touch each other (traditionally, they are served attached to each other, but it’s fine if they don’t!) Let cook for several minutes. Add 2/3 cup of water to the skillet and cover tightly. Cook for 5 minutes, adding more water if it evaporates before the 5 minutes is up.
Cook until water is evaporated and the gyoza are nicely browned on the bottoms, about 7 minutes total. Invert the gyoza onto a platter and place in the oven to keep warm. Repeat with remaining 2 TBS. of oil and gyoza. Depending on the size of your skillet, you might need to do a third batch, adding a little extra oil.
To make the dipping sauce, combine equal parts soy sauce and rice vinegar in a small bowl. Add a little splash of sesame oil, a sprinkle of Japanese chili pepper blend, and a little agave nectar for sweetness. Stir together.
Add water to dilute the sauce slightly. Taste and adjust amount of water or ingredients until it tastes right to you. It should be a nice balance of salty, sweet, and sour, with a little spice.
Serve the gyoza with a bowl of steamed rice and the dipping sauce on the side. Add a salad with sesame dressing and you have a comforting Japanese dinner!
