Peel and section grapefruit to get 1 1/2 cups sections, reserving 2 tablespoons grapefruit juice. Combine grapefruit, onion, and thyme in a large bowl.
Combine reserved grapefruit juice, vinegar, and next 4 ingredients; stir well with a whisk.
Add salad greens and dressing mixture to grapefruit mixture; toss well. Serve immediately.