Fill a covered bowl halfway with ice, add onion slices, and top with more ice. Add cold water until you're just able to get the lid on. Refrigerate for 3 hours.

Cut a slice off the top and bottom of each of the blood oranges, and slice downward to remove all skin and white pith. Be careful, as blood oranges will stain like beet juice. Cut oranges into 1/4-inch horizontal slices and remove seeds. Retain any juice and add to the juice of the navel orange. Put slices in a covered bowl or plastic bag with the olives and refrigerate for 3 hours along with onions.

Combine olive oil, balsamic vinegar, orange juice, sugar, mustard, oregano, pepper, and garlic powder in a jar with a lid, close, and shake well to combine. Refrigerate until 15 minutes before serving the salad. Remove from refrigerator to barely come to room temperature.

Drain onions and add to a large salad bowl. Add spinach, cherry tomatoes, onions, blood orange slices, and kalamata olives. Drizzle dressing on top and toss to combine. Sprinkle with blue cheese and macadamia nuts.