*Please see the note made in the introduction of the recipe about the cashew crema. Make the cashew crema by bringing a pot of water to a boil. Add raw cashews to the boiling water and cook for about 15 minutes. This should be long enough to soften the cashews. When the cashews are soft, drain away the water and transfer them to a food processor or powerful blender.
In the food processor or blender, pulse the cashews with coconut yogurt, lemon juice, and salt. Scrape down the walls of the food processor, and blend in the water starting with just 1 tablespoon at a time. You may not need all the water listed to make the crema. Continue adding water and pulsing until you have a loose crema that you can drizzle over the tacos later. At this point, you may choose to add more salt to taste. If you want a more sour taste (like sour cream), you may add white vinegar at your discretion. Set the cashew crema aside for later.
Next, thin slice the red onion and red bell pepper. Heat the olive oil in a pan over very high heat. When the oil just begins to smoke, add the onions and peppers to the pan. Let the veggies char on one side before gently tossing them as they continue to cook. Once the onions and peppers have gained golden brown edges and softened, set them aside for the end of the recipe.
Next, slice large portobello caps into ½-inch thick slices. You should have about 2 slices per taco. In a mixing bowl, toss the mushroom slices with the second amount of olive oil, salt, pepper, cumin, paprika, garlic powder, oregano, and cayenne. When the mushroom slices are seasoned, heat either the same pan from Step 3 or a new pan over high heat. When the pan is hot, place the mushroom slices in the pan.
Let the slices of portobello mushroom cook until charred in the pan. You want the spices charred on both sides. Watch for the mushrooms to reach smoking, release some excess moisture, and shrink slightly before flipping them to finish the cooking process. When you flip the mushroom slices, you can reduce the heat to finish cooking.
When the mushrooms are done, you can assemble your tacos. Cut away the lighter leaves and hard core from a head of lettuce. Select 1 leaf per taco, choosing the outer, larger, darker leaves to serve as your taco shells. To make a taco, arrange two slices of portobello mushroom with a fair portion of the cooked onions and peppers in a lettuce leaf. Drizzle some cashew crema over the taco with a fork or spoon. Repeat the assembly process until all the tacos are made.