To make the salad, toss the watermelon, black olives, red onion, and arugula together in a bowl. Put it in the fridge to chill. The contrast of a cold salad with the hot salmon is really refreshing.
To make the vinaigrette, toast the spices in a dry skillet over low heat for 1 minute, until they smell fragrant; shake the pan frequently to prevent scorching. In a spice mill or clean coffee grinder, grind the toasted spices with the sugar. Put the sherry vinegar and oil in a blender and add the ground spice mixture; give it all a whirl to blend. Season with salt and pepper.
Edited: October 15th, 2009