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 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.
Pre-heat oven at 400F. Generously spray a mini muffin tin with cooking spray.
Once the falafel mixture has chilled, use a small ice cream scoop or spoons to form the mixture into balls and place each ball in the mini muffin tray (roughly 3 Tablespoons per ball). (Note: You can also add additional flour at this point, if the mixture is too wet to scoop.)
Bake at 400F for 15 minutes.
Serve hot with Pita bread and a drizzle of Tahini Sauce.