For the Vinaigrette:
1/4 cup rice vinegar
1 tablespoon sugar
2 teaspoons prepared horseradish
1 teaspoon minced shallot
1/4 cup olive oil, more to taste
The first thing you need to do is:
Supreme the oranges and grapefruit: Using a very sharp knife, cut off the top and the bottom of the fruit, so it will sit flat on the cutting board. Starting where you see the orange and grapefruit separate from the white pith, cut away one section of peel and pith, following the line of the fruit. This will expose the underlying fruit. Continue cutting away sections of the peel and pith until only fruit remains. When you’re done, go back over the fruit, removing any traces of pith.
Working over a small bowl to catch the juice, slice the fruit into sections: Make a cut between the fruit and membrane, then do the same on the other side, freeing pure fruit. Repeat until you have removed all the fruit from the membrane. Squeeze any juice from the remaining membrane into the bowl. You will need about 2 tablespoons juice for the vinaigrette. Set the peeled fruit segments aside to use in assembling the salad.
Make the vinaigrette:
Add to the bowl, the juice, whisk in the vinegar, sugar, horseradish and shallot. Slowly whisk in the olive oil, emulsifying to form a vinaigrette. Taste and add one-half teaspoon salt, or as desired. This makes about 1 cup vinaigrette, which will keep, covered and refrigerated, up to 1 week.
Charred broccoli salad:
2 large stalks broccoli (1 generous pound)
1 tablespoon oil
1/2 teaspoon chile flakes, or to taste
2 supremed oranges
1 supremed grapefruit
1 shallot, shaved into thin rounds
1/3 cup crumbled cotija or feta cheese
Freshly grated horseradish root, for garnish
Heat a grill over medium-high heat until hot. Peel the broccoli stalks and cut the broccoli (stalks and florets) into large, bite-sized pieces. Toss the broccoli with the oil, a pinch of salt and the chile flakes. Grill the broccoli until lightly charred and crisp-tender, 8 to 10 minutes (the stalks will take longer to grill than the florets). Remove from heat.
In a large bowl, toss the broccoli with the orange and grapefruit segments and the shallots. Add 3 to 4 tablespoons vinaigrette, or enough to lightly coat, and gently toss it with the broccoli and citrus. Put the broccoli salad on a platter and garnish it with crumbled cotija or feta cheese. Grate the horseradish root over the salad, garnishing with about 1 teaspoon root, or to taste. Serve immediately.