1. Combine the vegetable oil, lemon juice, mint, coriander and salt in a bowl, and stir with a fork or a whisk until the ingredients are thoroughly blended. Stack the onions, beets and tomatoes in the following fashion:Arrange 6 of the onion slices side by side on a large serving platter and place a beet and a tomato on top of each.
2. Cover each stack with another layer of onions, beets and tomatoes.Sprinkle the chilies over the stacks, and sprinkle their tops evenly with the oil-and-lemon dressing. Marinate at room temperature for 30 minutes or in the refrigerator for at least an hour before serving.