Step 1

You may substitute tangelos or Cara Caras for the navel oranges. Valencia and blood oranges can also be used, but since they are smaller, increase the number of fruit to four.

Step 2

Cut away peel and pith from grapefruits and oranges. Cut each fruit in half from pole to pole, then slice crosswise into 1/4-inch-thick pieces. Transfer fruit to bowl and toss with sugar and ½ teaspoon salt. Set aside for 15 minutes.

Step 3

Melt butter in 8-inch skillet over medium heat. Add walnuts and ½ teaspoon salt and cook, stirring often, until lightly browned and fragrant, 2 to 4 minutes. Transfer walnuts to paper towel–lined plate.

Step 4

Drain fruit in colander, reserving 2 tablespoons juice. Transfer fruit to platter, arrange in even layer, and drizzle with oil. Whisk reserved juice, shallot, and mustard in medium bowl. Add arugula, ¼ cup raisins, and ¼ cup reserved walnuts and toss to coat. Arrange arugula mixture over fruit, leaving 1-inch border around edges. Sprinkle with remaining 1/4 cup raisins and remaining 1/4 cup reserved walnuts. Season with salt and pepper to taste. Serve immediately.