How to Make It
Heat canola oil in a large skillet over high. When oil shimmers, spread mushrooms evenly in pan. Cook until browned, about 1 minute. Stir mushrooms; spread evenly. Cook 1 minute. Remove from heat; stir in 1/8 teaspoon salt. Place mixture in a bowl; cool 5 minutes. Stir in 1/2 cup green onions and 3 minced garlic cloves.
Combine stock, oyster sauce, sesame oil, sugar, and 1/2 teaspoon salt in a small bowl. Gently combine turkey and pork in a large bowl. Add stock mixture. Using your hands, gently mix just until liquid is absorbed and mixture is sticky. Gently fold in mushroom mixture. Cover and chill up to 4 hours.
Heat olive oil over high in a large Dutch oven until you see wisps of smoke. Add remaining 4 minced garlic cloves; cook, stirring constantly, 15 seconds. Add ginger; cook, stirring constantly, until fragrant, about 15 seconds. Add 3/4 cup green onions; cook, stirring constantly, 15 seconds. Add tomatoes; bring to a boil, stirring often. Reduce heat to medium-low; stir in remaining 3/8 teaspoon salt. Cover and simmer 15 minutes.
Using your hands, gently shape turkey mixture into about 24 (1 1/2-inch) meatballs. Add meatballs to tomato sauce in an even layer. Adjust heat, if necessary, to ensure sauce is barely simmering. Cover and simmer until meatballs are just cooked through, 10 to 12 minutes. Transfer meatballs to a serving platter; cover with aluminum foil to keep warm.
Whisk together cornstarch and 1 tablespoon cold water in a small bowl. Add to tomato sauce; cook, whisking constantly, until sauce thickens, about 1 minute. Pour tomato sauce over meatballs. Garnish with remaining green onions.