Prepare the falafel mixture by combining 250 grams of soaked chickpeas, 1 chopped onion, 2 cloves of minced garlic, 1/4 cup chopped fresh parsley, 1/4 cup chopped fresh cilantro, 1 teaspoon ground cumin, 1 teaspoon ground coriander, 1/2 teaspoon red chili powder, 1/2 teaspoon baking soda, salt, and pepper in a food processor. Process until a coarse paste is formed.
Shape the falafel mixture into small balls or patties and refrigerate for 30 minutes.
Heat oil in a deep frying pan over medium heat. Fry the falafel balls until golden brown and crispy, about 3-4 minutes on each side. Drain on paper towels.
Prepare the tahini sauce by mixing 1/2 cup tahini, 2 tablespoons lemon juice, 1 minced garlic clove, salt, and 2-3 tablespoons of water in a bowl until smooth.
Warm 4 pita breads in a pan or oven for a few seconds on each side.
Assemble the sandwich by cutting the pita bread halfway to create a pocket. Stuff each pita pocket with 3-4 falafel balls, sliced tomatoes, cucumber, lettuce, and drizzle with tahini sauce.