Soak bamboo skewers in water first for about 30 minutes, to prevent them from scorching on the grill.
Meatballs: Working in batches, pulse bread into coarse crumbs in a food processor. Transfer to a bowl, drizzle with milk, and let stand 10 minutes.
Meanwhile, puree onion and garlic in processor until smooth. Add egg white, salt, and pepper; pulse to combine. Stir onion mixture into breadcrumbs. Add lamb and pork; gently mix until just combined. With dampened hands, form mixture into 1-inch-diameter balls. Place on a parchment-lined baking sheet, cover, and refrigerate until ready to use, up to 1 day. (Or freeze until firm, then transfer to resealable plastic bags and freeze up to 1 month; thaw overnight in refrigerator.)
Preheat grill for direct heat. Thread 2 meatballs onto each skewer and brush with oil. Grill meatballs, turning a few times, until just cooked through, 6 to 8 minutes.
Salsa verde: Stir together lemon zest and juice, salt, and pepper flakes in a bowl until salt is dissolved. Puree lemon mixture with herbs and oil in processor until smooth. Transfer to a bowl and cover with plastic wrap, pressing it directly onto surface. Store in refrigerator until ready to serve, up to 1 day; dollop onto warm meatballs.