Method

1. Remove the peel and white pith from the grapefruits.
2. Working over a large bowl to catch the juices and using a small sharp knife, cut the grapefruits between the membranes to release the segments into the bowl.
3. Squeeze the membranes into the bowl to extract as much liquid as possible.
4. Strain the grapefruit juice into a measuring cup and set the grapefruit segments aside.
5. You should have about 1/2 cup of grapefruit juice.
6. Add enough grapefruit juice to the accumulated grapefruit juice to equal 1 3/4 cups of juice total.
7. Simmer 1 1/2 cups of the juice in a heavy small saucepan over medium-high heat until it is reduced to 1/4 cup and has a thick syrup-like consistency, about 12 minutes.
8. Set the grapefruit reduction syrup aside.
9. Toss the lettuce leaves, fennel slices, and reserved grapefruit segments in a large bowl.
10. Divide the salad among 4 plates.
11. Melt the butter in a heavy medium saucepan over medium heat until it is a nutty brown, swirling the pan occasionally, about 2 minutes.
12. Add the capers, tarragon, and remaining 1/4 cup of grapefruit juice. Season the sauce to taste with salt and pepper.
13. Drizzle the sauce over the salads, then drizzle the grapefruit reduction syrup over the salads and serve.