Place beets in a large saucepan, cover with water, and bring to a boil. Simmer 1 1/2 hours, or until a fork is easily inserted. Drain, reserving 4 tbsp liquid. Rinse beets with cold water; let cool.
Cook potatoes in boiling water 20 to 30 minutes, or until just cooked. Drain, rinse with cold water; let cool. Peel beets and potatoes, halve, and slice thinly. Place in separate bowls.
Plunge tomatoes into boiling water for 1 minute. Transfer to bowl of iced water. Peel tomatoes, cut in halves, remove seeds, and dice flesh. Combine tomatoes and green onions with beets. Add garlic, parsley, cilantro, and reserved cooking water.
Make vinaigrette: Whisk vinegar, oil, salt, pepper, and cayenne pepper until combined. Stir two-thirds of the vinaigrette into beet mixture, and remainder into potatoes. Cover and refrigerate 30 minutes.
Just before serving, add more salt, pepper, and vinegar to taste. Spoon beet salad onto a platter and arrange potato salad around it. Garnish with olives.
Per serving: 381 calories, 5 g protein, 28 g total fat, 4 g saturated fat, 0 mg cholesterol, 27 g carbohydrates, 6 g fibre.