How to Make It
Combine beef, 1/3 cup of the onion, panko, cheese, 1 1/2 tablespoons of the oregano, fennel seed, 1 teaspoon of the garlic, 1/4 teaspoon of the crushed red pepper, and egg in a medium bowl. Shape mixture into 24 meatballs (about 1 tablespoon each).
Remove lid from a 6-quart Instant Pot®. Press [Sauté], and use [Adjust] to select "More" mode. When the word "Hot" appears, swirl in oil. Add remaining 1/3 cup onion and 1 teaspoon garlic; cook, stirring constantly, 3 to 4 minutes or just until tender. Stir in wine, scraping cooker to loosen browned bits. Stir in remaining 1 1/2 tablespoons oregano, remaining 1/4 teaspoon crushed red pepper, and sugar. Arrange meatballs in a single layer in cooker; top with tomatoes (do not stir). Turn cooker off.
Close and lock the lid of the Instant Pot®. Turn the steam release handle to "Sealing" position. Press [Manual]; select "High Pressure," and use [-] or [+] to choose 6 minutes pressure cooking time. When time is up, open the cooker using Quick Pressure Release (page 4).
While meatballs cook, cook spaghetti according to package directions, omitting salt and fat; drain. Serve meatballs and tomato sauce over spaghetti. Sprinkle with oregano leaves, if desired.