Preheat the oven to 425°F (220°C).
Lightly drizzle a large rimmed baking sheet with olive oil or line it with parchment paper and brush lightly with oil.
Add the ground beef and ground lamb to a large mixing bowl.
Add the salt, cumin, oregano, red pepper flakes, parsley, dill, mint, red onion, black pepper, and minced garlic.
Using clean hands, gently combine the ingredients until evenly distributed. Avoid excessively mixing or compressing the meat, which can make the finished meatballs dense.
Use a cookie scoop or tablespoon to portion the mixture into evenly sized pieces.
Gently roll each portion between your palms to form meatballs and arrange them on the prepared baking sheet, leaving a little space between each one.
Bake for approximately 15–20 minutes, or until browned and safely cooked through. Ground beef and lamb should reach an internal temperature of at least 160°F (71°C).
While the meatballs bake, prepare the tzatziki. Add the cucumber, garlic, and fresh dill to a food processor.
Pulse until finely chopped. For a thicker sauce, gently squeeze excess liquid from the cucumber mixture.
Transfer the cucumber mixture to a bowl and add the Greek yogurt.
Stir in the fresh lemon juice and season with salt to taste.
Refrigerate the tzatziki until you're ready to serve.
Serve the warm Greek Meatballs with Tzatziki Sauce generously spooned alongside or drizzled over the top.