Preheat oven to 400 degrees.
Peel butternut and cut in half, lengthways; scoop out the seeds and wash well, removing any bits of squash strings. Spread the seeds on a baking sheet to dry. Cut each half, top to bottom, into 3/8-inch-thick slices. Lay them on a large baking sheet. Brush with olive oil and sprinkle with salt. Place in the oven for 15 minutes or until tender when tested with the point of a knife. Remove from the oven and set aside to cool. Then, lower oven temperature to 300 degrees. Toss seeds with a drizzle of olive oil and a sprinkle of salt; spread on a baking sheet and roast for 15 minutes or until seeds start to pop.
Meanwhile, whisk together the crème fraîche, tahini, lemon juice, 2 tablespoons water and a pinch of salt. The sauce should be thick but runny enough to pour; add more water, if necessary. I always have to add at least 2 tablespoons, sometimes even more. Serve roasted squash on top of watercress; drizzle with crème fraîche sauce and sprinkle with squash seeds.