What's the ingredients in swedish meatballs?

2 Tablespoons olive oil , divided ▢1 onion , diced ▢1 pound lean ground beef ▢1 pound ground pork ▢1/2 cup breadcrumbs ▢2 large egg yolks ▢1/4 teaspoon ground allspice ▢1/4 teaspoon ground nutmeg ▢1 1/2 teaspoon kosher salt ▢3/4 teaspoon fresh ground black pepper