Make the tzatziki first — Grate the cucumber and squeeze out as much moisture as possible using a clean kitchen towel. Mix it with the Greek yogurt, garlic, dill, olive oil, and lemon juice. Season with salt and pepper, then refrigerate until ready to serve.
Mix the meatballs — In a large bowl, combine the ground beef, breadcrumbs, egg, garlic, red onion, parsley, oregano, cumin, salt, and pepper. Mix until just combined — don't overwork the meat.
Form the meatballs — Roll the mixture into balls about 1.5 inches in diameter. You should get roughly 20 meatballs.
Cook the meatballs — Heat olive oil in a large skillet over medium-high heat. Add meatballs in a single layer and cook for 10–12 minutes, turning every few minutes until browned on all sides and cooked through (internal temp of 165°F).
Serve — Plate the meatballs alongside a generous scoop of tzatziki, warm pita, and your favorite toppings.