To make the dressing, briefly pulse shallot in a food processor until chopped into small pieces.
To the shallots, add vinegar, marmalade, salt and pepper.
Blend until mixture is smooth.
With food processor on, slowly drizzle in oil until combined.
In a large bowl, combine the salad greens, grapefruit sections and avocado. Drizzle enough vinaigrette over greens to lightly coat. Toss, top with cheese and serve immediately.