Preheat the oven to 400°F. Line a baking sheet with parchment paper.
In a large bowl, combine ground beef or lamb, breadcrumbs, Parmesan cheese, parsley, onion, garlic, egg, oregano, salt, black pepper, cumin, and red pepper flakes (if using). Mix until just combined.
Form the mixture into meatballs, about 1 inch in diameter, and place them on the prepared baking sheet.
Bake in the preheated oven for 20-25 minutes, or until the meatballs are cooked through and browned.
While the meatballs are baking, prepare the tzatziki sauce. In a medium bowl, mix together Greek yogurt, grated cucumber, garlic, olive oil, lemon juice, salt, and pepper. Adjust seasoning to taste.
Once the meatballs are done, serve them warm with a generous dollop of tzatziki sauce on the side.