Grate the zucchini and squeeze it hard in a clean towel until it stops giving water. Wet zucchini makes a loose mix that will not hold a ball.
Mix the turkey, squeezed zucchini, egg, parmesan and half the salt with your hands, stopping the moment it comes together. Overworked mix bounces.
Shape into 18 balls and chill them 10 minutes; chilled meatballs hold their shape when they hit the pan.
Meanwhile, soften the onion in the oil, add the garlic and oregano for 30 seconds, then the crushed tomatoes and remaining salt. Simmer 10 minutes.
Lower the meatballs into the sauce, cover, and simmer gently 20 minutes, until they reach 165F at the center.
Uncover and simmer 10 minutes more. This stretch is not about safety; it is what takes them from cooked to genuinely tender.