Toss summer squash slices with olive oil, about one tablespoon of lemon juice, salt, pepper, and ground fennel and set aside. Toss onion slices with about two tablespoons of lemon juice, salt, and dried oregano and set aside. Combine the remaining ground fennel, remaining oregano, onion powder, and panko and set aside. Season the pork cutlets lightly with salt and pepper, then coat them with all purpose flour, the whisked egg, and the panko, successively. Add neutral oil to a frying pan or skillet over high heat. Once shimmering (around 350-380 F), lay the cutlets in the pan and fry on each side until deeply golden and crispy, about 6-8 minutes.
While the cutlets cook, combine mayonnaise, dijon mustard, one to two tablespoons of lemon juice (to taste), and green peppercorns (or, if using, capers), and fresh herbs, if using. Season with salt, taste, and adjust as needed.
When the pork is golden brown and cooked through, transfer to a rack and season with an extra sprinkle of salt. Assemble the sandwich: Add mayonnaise mixture to both inner sides of the focaccia slices. Lay marinated squash ribbons on the bottom slice, top those with the pork cutlet, and add quick-pickled onions on top. Close the sandwich and enjoy immediately.