Add the onion and garlic cloves to the bowl of a food processor and pulse just until they are finely minced.
Remove the mixture and press it through a strainer to release as much liquid as possible then set it aside.
Add the chickpeas, parsley, cilantro, salt, chili powder, and cumin to the bowl of the food processor and pulse until they are roughly blended but not pureed.
Return the onion mixture to the food processor, along with the baking powder and just enough flour so that when you pulse the processor, the mixture begins to form a small ball and is not sticky. (Note: Start by adding ¼ cup of flour, and then the remaining ¼ cup. You can add more if the mixture is still too wet.)
Transfer the falafel mixture to a bowl, cover it with plastic wrap and refrigerate it for 1 hour.
While the falafel mixture is chilling, prepare the tahini sauce by whisking together the yogurt, tahini and lemon juice. Season it with salt and pepper, cover it and place it in the fridge.
Once the falafel mixture has chilled, use a small ice cream scoop or spoons to form the mixture into balls (roughly 3 Tablespoons per ball) then flatten them slightly so they are the shape of patties. (Note: You can also add additional flour at this point if the mixture is too wet to scoop.)
Set a large sauté pan over medium heat and add a liberal amount of canola oil so that the pan is well-coated. Let the pan pre-heat for 3 minutes then add the falafel one by one, browning them on the first side for 3 minutes, then flipping them once and browning the second side until the mixture is cooked throughout.
Transfer the falafel to a paper towel-lined cooling rack and immediately season them with salt. Repeat this process until you have cooked all of the falafel.
Place three or four falafel inside a halved, warmed pita and drizzle with the prepared tahini sauce.