Combine nectarine, simple syrup, and lemon juice in a blender and purée until smooth. Use immediately or transfer to a container with a tightfitting lid and refrigerate for up to 2 days.
Combine amaretto, lemon juice, nectarine purée, and egg white in a cocktail shaker filled with ice. Shake vigorously until very chilled, about 30 seconds. Strain into a chilled cocktail glass and top with bitters.