Preheat oven to 425°F.
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 sauté to tender-crisp, 5-6 minutes.
Dress the watercress with lemon juice, EVOO, salt and pepper.