Add the stock to a large pan over medium heat and add the full block of tempeh. Allow to simmer for about half an hour.
Remove the tempeh from the pan and allow to cool
While the tempeh is cooking, place another large pan over medium-heat, and add the oil, diced onion, garlic, and pepper. Cook for about 10 minutes, stirring frequently, until the onion is fragrant and lightly browned.
Add the mushrooms and a pinch of salt. Cook until the mushrooms have softened and they have released their liquid, about 5 or 10 minutes
Crumble the tempeh and add it to the pan. Cook for about 5 more minutes, until the tempeh has browned and absorbed any mushroom liquid left in the pan. If the mushrooms don’t get very juicy, use a little of the stock that the tempeh was simmered in.
While the tempeh and mushrooms are cooking, mix together the tomato sauce, maple syrup, Worchester sauce, mustard, garlic powder, onion powder, and red pepper flakes, if using.
Add the tomato mixture to the pan and stir to coat the tempeh and vegetables. Reduce the heat to medium, and simmer uncovered for about 10 minutes, stirring frequently, until the tomato sauce heats throughout and reduces slightly.
Spoon the mixture onto buns and serve hot.