Smashed Edamame Falafel Pitas

For the quick-pickled onions (optional):
0 Finely grate the zest of 1/2 a lemon into a medium bowl. Juice half the lemon into the bowl (reserve the other half). Add the sliced onions and a pinch of kosher salt and toss to coat. Set aside to pickle, tossing occasionally.
For the falafel:
1 In the bowl of a food processor fitted with the blade attachment, add the red onion, edamame, parsley, garlic, flour, salt, cumin, coriander, baking powder, and cayenne (if using). Squeeze in the juice from the reserved lemon half. Process until mostly smooth, scraping down the bowl as needed. Season with more salt to taste. 
2 Divide the mixture into four portions (about a heaping 1/2 cup each) and shape into balls. Very carefully split the pita pockets into top and bottom halves. I find it easiest to slice off the very tip of the pita, then use that opening to begin slicing.
3 In a large nonstick or well-seasoned cast iron skillet, heat 1/2 tablespoon of oil over medium-high heat. Add one of the falafel patties, top with one of the pita halves (with the split half touching the falafel), and smash with a burger press, small heavy skillet, or the lid of a small stainless steel pot. Press firmly until the falafel spreads nearly to the edges of the pita. The thinner the falafel layer, the crispier it will become. 
8 Cook until the falafel is lightly golden brown and the edges of the pita are crisp, about 3 minutes. Flip and cook for about 1 minute more. Transfer to a platter and set aside.
4 Repeat the remaining oil, falafel mixture, and pita halves.
5 Top each pita with desired toppings such as pickled red onions, lettuce, cucumber, tomatoes, and tahini.