Coat the cauliflower steaks with the olive oil and either pan fry, roast at 400 degrees for about 10 minutes, or grill.
Combine the tomatoes, onions, parsley, black beans, lime juice, and cumin in a bowl. Season with salt and pepper, to taste. Add more lime juice, if needed.
To serve, place the cauliflower steak on a plate, and generously spoon the black bean relish on top. Garnish with sliced avocado, if desired.