Soak 225g of dried chickpeas in plenty of cold water for at least 12 hours, then drain well. Do not use canned chickpeas.
Process the chickpeas with 1 cup of parsley leaves, 1 cup of cilantro leaves, 6 scallions, using only the white and light green parts, and 2 garlic cloves until finely chopped.
Add 1 teaspoon of cumin, ½ teaspoon of ground coriander, 1½ teaspoons of salt, ½ teaspoon of baking powder, 4 teaspoons of chickpea flour, and 5 tablespoons of water. Process to fine, moist grains that hold together when squeezed.
Shape heaping spoonfuls into balls about 1in / 2.5cm wide. Chill for 30 minutes.
Heat 500ml of vegetable oil in a small, deep pan to 350°F / 180°C; it should be at least ¾in / 2cm deep. Fry in batches for about 4 minutes, turning until deep golden. Drain on paper towels.
Mix 4 tablespoons of tahini, 2 tablespoons of lemon juice, and ¼ teaspoon of salt. Gradually stir in 4 tablespoons of water to make a smooth sauce. Serve with the falafel.
