Add mayonnaise, sour cream, lemon juice, cilantro, jalapeños, scallions, garlic, salt and pepper to a blender. Blend on high until the mixture is smooth, and set the sauce to the side.
½ cup mayonnaise, ¼ cup sour cream, 1 lemon, 1 cup cilantro, 2 jalapeños, 2 scallions, 2 cloves garlic, ½ teaspoon fine sea salt, ¼ teaspoon fresh ground black pepper