Wrap the beets in foil and place on a rack, set over a pan of hot water. Roast in a preheated 400 degree oven for 40 minutes. When cool enough to handle, peel and slice 1/4 inch thick. Mix together the onions, vinegar, sugar, fennel seeds, chile, salt, and pepper. Mix into the beets and marinate, refrigerate for at least 4 hours. Garnish with Thai basil chiffonade.