Brush the nectarines with the amaretto. Grill until golden, tender and you have achieved beautiful grill marks, about 2 to 3 minutes on each side, 6 minutes total. Remove from the grill and let cool slightly.
Add the mascarpone, whipping cream, sugar, lemon zest, and vanilla to a large mixing bowl and beat with a hand mixer until soft peaks appear.
Plate the grilled nectarines on a platter and top each with a dollop of whipped mascarpone and a sprinkling of chopped pistachios. Serve immediately.