Whisk together the oil, lemon juice, mint, cilantro, and salt until thoroughly blended. Stack the onions, beets, and tomatoes on a serving platter or individual salad plates in the following manner: place a slice of onion on the plate, place a slice of beet on top of that, and a slice of tomato on top of that. Repeat. Sprinkle with the optional hot green chilies, and drizzle the dressing over the top. Allow to marinate at room temperature for 30 minutes, or in the refrigerator for at least 1 hour before serving.