Heat a grill pan over medium-high heat.
In a small bowl, combine 1 teaspoon honey and 1 teaspoon water. Add the quartered nectarines and gently toss to coat.
Brush the grill with a little olive oil. Add the nectarines, cut sides down, in a single layer. Grill for about 2 minutes a side or until grill marks are present and fruit has softened a bit. Remove from the grill, allow to cool to the touch, then slice each quarter into 3 or 4 wedges.
In a small bowl, mix together the mascarpone with about 2 tablespoons honey. Set aside.
Increase grill pan heat to high. Brush both sides of the bread slices with olive oil and place on the grill to toast for 1 minute each side. Remove to a plate.
Spread a generous amount of the honey-mascarpone mixture (about 1 heaping tablespoon) on each slice of toasted bread. Top each slice with 3 or 4 nectarine wedges, then add some chopped pistachios, if using.
Plate the bruschetta on a serving platter. Drizzle the entire dish with remaining honey and balsamic. Top with chiffonade basil and serve.