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. Save the stock for later use.
Add the onion and oil to a large pan over medium-high heat. Cook, stirring frequently, until the onion begins to brown and become fragrant, about 5 minutes. Add the garlic and cook for another minute or two.
Whisk together the tamari, tomato paste, spices, and ½ cup of the reserved vegetable stock
Crumble the tempeh and add it to the pan along with the vegetable stock mixture. Mix everything well.
Continue cooking until the mixture is heated throughout, about 5 more minutes. If the mixture gets too dry, add a little more reserved vegetable stock.
To serve the tacos, fill each tortilla with the tempeh mixture along with lettuce, diced avocado, diced tomato, jalapeño, and hot sauce, if using. Serve hot.