Preheat the oven to 350°. Spread the pistachios on a rimmed baking sheet and toast in the oven for about 8 minutes, or until lightly browned; let cool.
In a large bowl, combine the lamb, beef, pistachios, onions, cumin, salt, black pepper and crushed red pepper and mix with your hands. Refrigerate for at least 2 hours or overnight.
Lightly knead the parsley into the meat. Roll the meat into tablespoon-size balls. Heat 1 tablespoon of the oil in a large skillet. Add half of the meatballs and cook over moderate heat until browned all over, about 8 minutes; reduce the heat if the meatballs brown too quickly. Drain on paper towels. Repeat with the remaining 1 tablespoon of olive oil and meatballs. Serve the meatballs hot or at room temperature with the Tahini Sauce.