Spiced Lamb Meatballs with Walnut Romesco

Name: Spiced Lamb Meatballs with Walnut Romesco
Ingredients: 3 tablespoons chopped walnuts; 1 small red bell pepper, halved, seeds removed; 1 medium tomato, cored; 1 garlic clove, unpeeled; 1 tablespoon olive oil; 1 tablespoon sherry vinegar; ½ teaspoon chipotle chile powder or chili powder; 1½ teaspoons finely chopped parsley; ½ teaspoon sweet paprika; Kosher salt, freshly ground pepper; ½ teaspoon coriander seeds; 1 teaspoon cumin seeds; ½ medium onion, chopped; 1 cup (firmly packed) cilantro leaves with tender stems; 1 cup (firmly packed) parsley leaves with tender stems; 2½ teaspoons kosher salt, divided; 1 large egg yolk; 1½ teaspoons paprika; 1 teaspoon freshly ground black pepper; ⅛ teaspoon cayenne pepper; 1 pound ground lamb; 2 tablespoons olive oil; A spice mill or a mortar and pestle