Combine sliced summer squash with olive oil, balsamic vinegar, salt and pepper and set aside. Meanwhile, mix mayo with mint, parsley, basil, cilantro, and the juice from 2 lemon wedges - set aside.
Over a hot grill, cook summer squash until al dente - just enough so that the squash isn't soggy but is still cooked through.
Slice open baguette and smear both sides evenly with the herb-packed mayo. Next, add a layer of the grilled summer squash and evenly distribute the pickled red onions.
Next, crumble the feta over the squash and onions so that each potential bite looks perfect.
Gently close the baguette and cut it into desired sandwiches.
You can typically get anywhere from 4-6 sandwiches depending on the baguette size and how hungry you are.