In a large bowl, soak the bread in the milk until very soft, 5 minutes. In a medium skillet, melt 1 tablespoon of the butter in 1 tablespoon of the oil until shimmering. Add the onion and cook over low heat, stirring, until softened, 5 minutes. Add the cumin and cayenne and cook until fragrant, 1 minute. Let cool.
Add the ground pork and beef to the soaked bread. Gently mix in the cooked onion, celery salt, kosher salt and beaten egg until thoroughly combined. Refrigerate until firm, about 30 minutes.
Shape the meat mixture into 1-inch meatballs. In a large skillet, melt the remaining tablespoon of butter in the remaining 2 tablespoons of oil. Add the meatballs and cook over moderately high heat, turning occasionally, until browned and cooked through, about 8 minutes; reduce the heat to moderately low if the meatballs begin to brown too quickly. Drain on paper towels.
Pour off the fat from the skillet, add the cream and scrape up any browned bits from the bottom of the pan. Simmer over moderately low heat until reduced by one-third, about 4 minutes. Season with salt and black pepper. Return the meatballs to the sauce and simmer for 2 minutes before serving.
