In a large bowl, combine ground meat, grated onion, garlic, parsley, breadcrumbs, egg, oregano, cumin (if using), salt and pepper; mix gently until just combined.
Shape the mixture into even meatballs about 1½ inches (4 cm) in diameter. Wet your hands if needed to prevent sticking.
Heat 3 tbsp olive oil in a large skillet over medium-high heat until shimmering.
Add meatballs in batches, turning to brown all sides; don't worry if they aren't fully cooked inside.
Transfer browned meatballs to a plate while preparing the sauce in the same skillet; pour off excess fat if necessary.
Add crushed tomatoes, water or stock and a pinch of sugar to the skillet; stir and bring to a gentle simmer.
Return meatballs to the sauce in a single layer, spooning sauce over them to coat.
Cover and simmer on low for 20–25 minutes until meatballs are cooked through and the sauce has thickened slightly.
Taste and adjust seasoning with salt, pepper and a splash of lemon juice if desired.
Serve hot topped with crumbled or cubed feta and extra chopped parsley; garnish with lemon wedges if liked.