Make the dipping sauce: in a medium bowl, add the rice vinegar, soy sauce, garlic, ginger, green onions, sesame oil, and crushed red pepper flakes and whisk to combine.
Heat oil in a large pan or wok over medium heat. When the oil is shimmering, add the onion, ginger, and garlic. Cook until the onion is translucent, stirring occasionally, about 4 minutes.
Add the asparagus, carrots, peas, and cabbage, and stir. Season with salt and pepper, and cook for 3-4 minutes, until the vegetables are softened.
Add the spinach and cook until wilted. Remove the filling from the heat and transfer to a large bowl to cool to room temperature.
Once the filling is cooled, add the chives, soy sauce, and sesame oil, and stir until well combined.
Place 1 tablespoon of the vegetable mixture onto a gyoza wrapper. Dip your finger in water and run it around the edge of the wrapper.
Fold in half and pinch together, pleating the dough as you go.
Heat 1 tablespoon of oil in a large skillet over low heat and add the dumplings. Cook for 3-4 minutes, until a crust starts to form on the bottom. Pour ¼ cup (60 ml) of water in the pan and cover with a lid. Steam for 6-8 minutes, then remove the dumplings from the pan.