Mix the sauce in the slow cooker: Add crushed tomatoes, broth, tomato paste, lemon juice, oregano, salt, red pepper flakes, and olive oil.
Stir to combine.
Make the meatball mixture: In a large bowl, combine turkey, breadcrumbs, feta, red onion, garlic, egg, parsley, dill, oregano, salt, pepper, and lemon zest. Add 1 tablespoon milk or water if the mixture feels dry.
Shape the meatballs: With damp hands, form 1 1/2-inch balls (about golf-ball size). You should get roughly 24–28 meatballs.
Optional sear for extra flavor: Heat a splash of oil in a skillet over medium-high.
Brown meatballs on two sides, 1–2 minutes per side, just to get color. Do not cook through. This step adds depth but isn’t required.
Nestle into the sauce: Gently place meatballs into the slow cooker, submerging most of each meatball in sauce.
Cook low and slow: Cover and cook on Low for 4–5 hours or on High for 2–3 hours, until meatballs reach an internal temperature of 165°F and are tender.
Taste and finish: Stir gently.
Adjust salt, add a squeeze of lemon if you like it brighter, and sprinkle with fresh herbs.
Serve: Spoon meatballs and sauce over rice or orzo, stuff into pitas with cucumbers and tzatziki, or plate with a big Greek salad. Top with extra feta and herbs.