Adding cinnamon gives these meatballs a slight Moroccan feel. You can make this a day ahead and reheat on the stovetop before the party. Using different types of beef adds moisture to the meatballs while keeping sat fat in check.
Halve onion lengthwise. Finely grate 1 onion half to equal 1/4 cup. Finely chop remaining onion half to equal 1/2 cup; set aside. (Reserve any remaining onion for another use.)
Combine sirloin, ground beef, panko, 1/4 cup grated onion, 1/4 cup parsley, oil, 1 1/4 teaspoons garlic, cumin, 1/4 teaspoon salt, and 1/4 teaspoon black pepper in a large bowl; use hands to gently combine. Shape beef mixture into 36 small meatballs (about 1 tablespoon each).
Heat butter in a large skillet over high. Add meatballs, and cook, turning occasionally, until lightly browned, 6 to 7 minutes. Transfer to a plate lined with a paper towels.
Reduce heat to low; add crushed red pepper, cinnamon, 1/2 cup chopped onion, remaining 1 1/4 teaspoons garlic, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon black pepper; cook, stirring often, until onion mixture is softened, about 5 minutes. Add tomato paste; cook, stirring constantly, until onion mixture is well coated, 1 minute. Add crushed tomatoes and sugar. Bring to a boil over medium-high; add meatballs to sauce, and cook until just cooked through, 4 to 5 minutes. Top with remaining 1/4 cup parsley.