1. Remove the green outer leaves from the frisée and discard or reserve for another use. Trim off any dark green tips and cut off the root ends (you should have about 5 cups).
2. Finely grate 1 teaspoon zest from one of the grapefruit. Trim the entire peel from all of the grapefruit and then cut the segments free from their membranes.
3. Combine the grapefruit zest, shallot, vinegar, lemon juice, and a pinch of salt in a small bowl.
4. Let sit for 5 to 10 minutes.
5. Whisk in the olive oil.
6. Taste with a leaf of frisée and season the vinaigrette with more vinegar or salt if necessary. Set aside.
7. Arrange the avocado slices on a platter or distribute among individual serving plates. Season with salt.
8. Put the frisée in a large bowl and season with salt and pepper.
9. Gently toss with just enough vinaigrette to lightly coat the greens.
10. Taste and season with more salt if necessary.
11. Gently scatter the frisée on and around the avocado (the salad will be somewhat flat).
12. Nestle the grapefruit slices into the salad.
13. Drizzle more vinaigrette on the salad, focusing on the avocado; you may not need it all.
14. Sprinkle with the cilantro and serve.
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