The most beautiful ruby-red grapefruit is available in December; use it to make this jewel-like salad. In other seasons, the grapefruit will be smaller and you will need more.
Working over a bowl to catch the juices, use a paring knife to slice between the sections and membranes of each grapefruit; remove the segments whole. Place each segment in bowl as completed. Repeat process with oranges in same bowl. Transfer to refrigerator. Recipe may be made up to this point 1 hour ahead.
Make vinaigrette: In a small bowl, combine vinegar, honey, and salt and pepper to taste. Slowly whisk in olive oil, then canola oil, until mixture is well combined. Set vinaigrette aside.
Just before serving, arrange radicchio and endive leaves on a large serving platter. Arrange grapefruit and orange segments in the center of the platter. Scatter red-onion slices over the top. Drizzle the reserved vinaigrette over the salad, and serve.