Prep and mix. Preheat the oven to 400°F (200°C). Line a rimmed baking sheet with parchment paper. In a medium bowl, combine the lamb, egg, onion, flour, garlic, parsley, mint, oregano, cumin, lemon zest, salt, and pepper. Using your hands, mix the ingredients together until well combined.
Make the lamb meatballs. Measure out 1 ½ tablespoons of the mixture (or use a medium cookie scoop) and roll the mixture between your hands, forming about 30 meatballs. Place the meatballs on the prepared baking sheet.
Bake the lamb meatballs. Bake the meatballs in the oven for 25 to 30 minutes, until browned on the outside and cooked through.
Make the mint chimichurri. While the meatballs are baking, make the mint chimichurri by combining the mint, parsley, garlic, vinegar, salt, and red pepper flakes in a mixing bowl. Slowly stir in the olive oil until the sauce is loose and spoonable. Add black pepper to taste. Let the sauce rest for 10 to 15 minutes to allow the flavors to meld.
Serve. Serve the lamb meatballs with the mint chimichurri spooned on top or served alongside.