In a bowl, mix together all the ingredients except the oil. Cover with plastic wrap and refrigerate for about 2 hours.
With the rack in the middle position, preheat the oven to 180°C (350°F).
Turn up the edges of a sheet of aluminum foil. Place the onions on the foil and sprinkle with the vinegar and sugar. Season with salt and pepper. Crimp together the edges of the foil to make a well-sealed package. Place on a baking dish and bake until the onions are tender, about 45 minutes. Keep warm.
PURÉED BLACK BEANS
In a food processor, purée the beans, lime juice and oil. Season with salt and pepper. Transfer to a serving bowl.
In a grill pan, heat the oil over medium-high heat. Grill the pork on both sides. Cut into thin strips. Arrange on a platter with the onions. Place the platter in the centre of the table with the garnishes.
To serve, spread bean purée on a hot or cold tortilla. Garnish with meat, tomatoes, onions, sour cream and cilantro.