To make the chipotle crema, finely chop the chipotle chiles in adobo sauce. Mix together the chiles with sauce with 1 cup of sour cream. (You can adjust quantities of pureed chiles and sour cream to your taste).
Toast the buns and set aside. Have all of your other fixings prepared. Wait to slice your avocado until just before you are ready to assemble your sandwiches and squeeze the juice of 1/2 a lemon over it.
Season pork with salt and pepper. Put flour into a wide shallow dish and set aside. Lightly beat eggs in a second wide shallow dish and set aside. Put bread crumbs into a third wide shallow dish and set aside. Dredge one piece of pork at a time in flour, shaking off excess, then dip into egg, evenly coating each side, then dredge in bread crumbs; set aside.
Heat the oil (or oil and butter) in a large heavy-bottomed skillet over medium-high heat until hot but not smoking.
Cook two pieces of pork at a time, turning once, until golden brown, about 2 - 3 minutes per side. Drain on paper towels. Cook the remaining pieces the same way.
To assemble sandwiches, distribute the lettuce on the bottom halves of each bun, followed by the schnitzel, avocado slices, onion slices, grated cheese and chipotle crema and cover with the bun tops.