Make sauce: In a small bowl, stir yogurt with lemon zest, mint, olive oil, salt, and garlic. Cover and chill until ready to serve.
Make meatballs: In a large bowl, combine ginger, garlic, ras el hanout, salt, and pepper. Add lamb and beef and gently mix just until seasonings are distributed.
In your palms, gently roll meat to form 1 1/2-in. balls; set on a rimmed baking pan.
Preheat oven to 200°, line another rimmed baking pan with paper towels, and put it and a heatproof serving bowl in the oven to keep warm. Heat a 12-in. frying pan over medium heat. Add oil and swirl to coat. Brown meatballs in 2 batches on all sides, turning as needed, 10 to 12 minutes for medium-rare. Transfer meatballs to the paper towel–lined pan.
Transfer meatballs to the warm serving bowl, garnish with mint sprigs, and serve with yogurt sauce.
*Buy at well-stocked supermarkets.
Make ahead: Shape, wrap, and chill meatballs up to 1 day (their texture will be a little softer than if just made).