Slice the mushrooms and add them to a medium-sized pot with 2 cups of water. Bring to a boil, then simmer for 20 minutes until the mushrooms are tender. Set aside.
Grab a medium-sized bowl, and add 1 cup of the cooked, cooled oats
Peel and roughly chop an onion, chop it finely in a food processor, then add the onion to the bowl with the oats
Add the black beans to the food processor, grind coarsely, and add to the bowl with the oats and onions.
Squeeze the water out of the mushrooms then add the mushrooms to the food processor along with the ground flaxseed, the butter or margarine, and 2 tablespoons of the mushroom broth. Pour the mushroom mixture into the bowl with the other ingredients
Add the spices and seasonings to the mixture and combine thoroughly.
Transfer the haggis mixture to a heatproof bowl and wrap tightly with saran wrap and tinfoil.
Place a rack inside of a large pot, place the haggis on the rack, and fill the pot with 1-2 inches of water. Place the pot on the stove with the lid on on high heat until the water boils. Reduce the heat to a low simmer and cook the haggis in the steamy simmering environment for 2 hours.
Enjoy the haggis piping hot right away, or chill in the fridge to firm up the haggis so it can be sliced, formed into patties, and fried until crispy for breakfast sandwiches. YUM.