Rinse quinoa well under cold water and cook according to package directions. Drain and let it cool.
Peel the skin and white membrane off the grapefruit and separate the pieces. Reserve three grapefruit slices for the vinaigrette and squeeze the juice; set aside. Cut up the remaining grapefruit into small pieces for the salad.
In a medium bowl whisk the olive oil, grapefruit juice (from 3 slices of grapefruit), vinegar, chopped shallots, salt and pepper.
In a large bowl toss spinach, quinoa, avocado, grapefruit and vinaigrette.