2 pink grapefruits
1/2 teaspoon salt
1/4 teaspoon red-pepper flakes
1/4 cup olive oil
1 head fennel
1/4 red onion
1 cup torn arugula
4 ounces lump Dungeness crabmeat
Using a very sharp knife, cut off top and bottom of the grapefruit, so it sits flat. Starting where pink flesh separates from white pith, cut away peel and pith, following line of fruit to expose it. Cut away sections of peel and pith until only fruit remains.
Working over a small bowl to catch juice, slice fruit into sections, making a cut between fruit and membrane; repeat on other side, freeing pure fruit. Repeat until all fruit is freed from membrane. Squeeze any juice from membrane into bowl (you will need about 2 tablespoons). Whisk in the salt, pepper flakes and oil.
Quarter the fennel lengthwise and remove triangular core at center. Using a mandoline or very sharp knife, slice fennel as thin as possible, about 1/8 inch, to yield about 3 cups, lightly packed. Repeat with red onion to yield about 1/2 cup.
Combine fennel and red onion in a large bowl and toss with enough dressing to lightly moisten and arrange on a platter in a low mound. Add the arugula to same bowl and toss, adding 1 teaspoon vinaigrette if needed. Arrange arugula on top of fennel.
Add grapefruit and the crabmeat to same bowl and toss very gently to avoid breaking up citrus or crab. You want it barely moistened; if needed, add 1 teaspoon vinaigrette. Arrange on top of arugula and serve immediately.
Makes 4 servings.