Preheat oven to 425F.
Combine ingredients for the Maple BBQ sauce in pot and bring to a bubble then reduce heat to low and simmer gently 20-25 minutes to thicken.
At the same time, heat grill pan or cast-iron skillet. Season the tenderloin well with salt and pepper, and drizzle with EVOO. Cook over medium-high heat to brown all over then transfer to oven to cook through, 20-25 minutes.
Melt butter in a skillet over medium heat and add apples. Season with nutmeg; add onion, salt and pepper then saut to tender-crisp, 5-6 minutes.
Dress the watercress with lemon juice, EVOO, salt and pepper.
Slice pork, arrange on rolls and top with lots of sauce or toss the pork slices in the sauce before piling on rolls. Melt cheese, if using, over the top of pork under a hot broiler then top with apples and onions, and watercress, and set bun tops in place.