Keep the peel on the fruits to start, which makes slicing them into thin pieces much easier. With a sharp knife, slice the grapefruits and the oranges into thin rounds. Remove the peel from the perimeter of all the rounds. (Use your finger or make a small slit into the peel and continue to tear the peel off. You’ll want to be delicate with this entire process so as not to damage the round shape.)

In a small bowl, whisk together the olive oil, vinegar, salt, sage, and pepper to taste.

Arrange the fruit rounds on a large platter or plate, alternating the colors. Drizzle with the vinaigrette, and dust with the cinnamon just before serving.

Serve immediately, or store in an airtight glass container in the fridge for 1 day.