1. Place pork slices in a single layer between two sheets of plastic wrap and pound to 1/8-inch thickness (use flat side of meat mallet or a rolling pin). Press any bacon pieces that have fallen off back onto the cut side of the pork slices.
2. Working in 2 batches, heat 1 tablespoon oil in a large nonstick skillet over Medium-High. Cook half the pork 3 minutes on each side until internal temperature reaches 145°F to 160°F. Set aside on separate plate and repeat with remaining 1 tablespoon oil and pork.
3. Spread guacamole evenly over 4 slices of toasted bread. Spread mayonnaise evenly over the remaining 4 slices. Layer onion, lettuce, 2 slices tomato, and 2 slices pork on top of guacamole. Close sandwiches with mayonnaise topped bread. Press down gently and cut sandwiches in half.