Make the meatballs: Combine all meatball ingredients except olive oil in a large bowl and mix well to combine. Form mixture into approximately 1” balls, and place them on a baking sheet lined with parchment paper (you should end up with about 15-16 meatballs).
Heat the olive oil in a large skillet over medium heat until hot, then add the meatballs to the hot pan. Cook 2-3 minutes a side, or until surface begins to brown, until meatballs are browned on all sides. Transfer meatballs to a plate and set aside while you start the sauce.
Drain out all but 1 Tbsp oil from the skillet. Return to medium heat, then add the onion and garlic, cooking until just beginning to soften, about 3 minutes.
Add the harissa paste and cook 1 minute more. Add the lemon juice and tomatoes, and cook, stirring frequently, for another 3-4 minutes. Season sauce to taste with salt. (note: when I used unsalted tomatoes and a harissa paste that wasn’t too terribly salty, ½ tsp salt was the perfect amount to season the sauce, but this may vary depending on what you use.)
Stir the meatballs back into the sauce, cover skillet, and let cook over medium heat until meatballs are cooked through, about 8-10 minutes.
Remove cover from skillet, and give everything a good stir. If the sauce seems too watery, give it another minute or two to cook without the lid and it should thicken up a bit. Remove pan from heat, sprinkle feta and mint evenly over top, and serve.