Prep the falafel: Put all ingredients into a food processor and blend until combined but not completely smooth. Chill in the refrigerator for at least 30 minutes.
Make the falafel: Heat 1 inch of olive oil in a large pan over medium heat. Remove falafel mixture from the refrigerator, and gently place heaping tablespoons of the mixture into the oil. Allow the falafel to brown on one side, about 3-4 minutes, and then use a slotted spoon to flip them over for another 3-4 minutes. Transfer to a paper towel-lined plate.
Tahini Sauce
Combine all of the ingredients together in small bowl. Set aside.
Pickled Onions
Thinly slice the onion. Place the slices into a mason jar or heavy duty plastic container.
Bring the water, vinegar, sugar and salt to a boil in a small saucepan. Once the salt and sugar are dissolved, pour the pickling liquid over the onions. Fill to the top of the container and seal it. Let it sit until it cools to room temperature. Store in the refrigerator.
Assemble the Pita Sandwiches
Spread the inside of a pita pocket with tahini sauce. Fill the pita with falafel patties, one leaf of romaine, cucumbers, tomatoes and onions. Top the pita with additional tahini sauce, if desired.