Cut tempeh into 1/2-inch cubes. Set aside.
Place onions in a large non-stick skillet over medium-high heat. Cover the skillet. Watch the onions as they cook, stirring often. After a few minutes, the onions may start to stick. If this happens, add water, one tablespoon at a time. Cook just until the onions soften and start to brown around the edges, about 5 minutes.
Add minced garlic and chopped mushrooms to the skillet and lower heat to medium. Cook until the mushrooms soften, about 3 minutes.
Add the tempeh cubes and all the remaining filling ingredients except the lemon juice and tortillas. Turn the heat down to low and simmer for 10 minutes, adding water 1 tablespoon at a time if filling is sticking. Stir occasionally.
When filling is heated through and fragrant, add lemon juice. Taste and adjust seasoning as needed.
Cool slightly before assembling tacos.
While filling is cooking, add peppers to a non-stick skillet. Heat on medium-high until peppers start to blacken, stirring often. Turn heat to low and cook until soft, stirring occasionally. Set aside.
Mix the onions, parsley and sumac while filling is cooking. Set aside.
To assemble the tacos, place about 2 tablespoons of filling in a tortilla. Add garnishes of choice.
Makes 12 tacos
*Note: Used extensively in the Middle East, sumac is generally safe to eat but may cause allergic reactions for certain people.
Sumac is available online or in stores that sell Middle Eastern food products.
Do not forage for your own sumac because it's difficult to differentiate between sumac and poison sumac. An acceptable substitute is lemon juice.