Make the meatballs: Preheat the oven to 400 degrees F and line a baking sheet with parchment paper.
Heat 1 tablespoon vegetable oil in a large nonstick skillet over medium-high heat.
Add the cabbage and season with 1/4 teaspoon salt; cook, stirring occasionally, until softened, about 4 minutes.
Transfer the cabbage to a plate to cool. Wipe out the pan, then add the remaining 1 tablespoon vegetable oil and the mushrooms. Season with 1/4 teaspoon salt and a few grinds of pepper and cook, stirring occasionally, until lightly browned, about 5 minutes.
Transfer the mushrooms to the plate with the cabbage to cool.
Lightly beat the eggs and egg white in a large bowl.
Add the pork, scallions, garlic, ginger, soy sauce, sesame oil, sugar and cornstarch.
Add the cabbage, mushrooms and a few grinds of pepper and mix with your hands until just combined (do not overmix). Dampen your hands and shape the meat mixture into 18 balls (about 2 inches each); arrange on the prepared baking sheet.
Mix the hoisin sauce, Sriracha, vinegar, sugar and 1 tablespoon water in a bowl; set aside 1/2 cup for serving.
Brush the meatballs with the remaining sauce and sprinkle with the sesame seeds.