Line a baking sheet with a large piece of plastic wrap, overhanging on all sides. Use two pieces if your wrap is not wide enough.
Slice focaccia through the middle and place the bottom on the plastic wrap, cut side up. Place the top on the cutting board, cut side up.
In a small bowl mix together the mayonnaise and pesto. Spread evenly on both the top and bottom pieces of bread.
Layer on the genoa and ham, overlapping slices as needed to fit. Top with roasted red pepper slices, then overlapping slices of cheese. Add the basil leaves, then place on the top piece of bread.
Bring up the sides of the plastic wrap and wrap the sandwich tightly. Set a heavy pan on top of the sandwich, and transfer the stack to the refrigerator. Let rest for at least 8 hours, or preferably 24 hours.
When ready to serve, unwrap sandwich and cut into 8 pieces for meal-size sandwiches, or 16 pieces for snack sandwiches.