Preheat oven to 400 degrees. Line a rimmed 18 by 13-inch baking sheet with aluminum foil, spray with non-stick cooking spray.
Add bread crumbs, nutmeg, allspice, cardamom, 1 tsp salt (or to taste), 1/2 tsp pepper to a large mixing bowl. Pour in milk and toss with a fork, let rest while sautéing onions.
Melt 1 Tbsp butter in a 12-inch skillet over medium heat. Add onions and saute until softened and slightly golden brown, about 6 minutes, while adding garlic during the last 30 seconds.
Add beef and pork to bread crumb mixture along with egg yolks and onion mixture. Gently toss mixture to evenly distribute ingredients.
Scoop dough out 1 oz at a time (using either a medium cookie scoop or nearly 2 tbsp each) and shape into balls. Transfer to prepared baking sheet spacing evenly apart.
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