How to Make It
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.