Remove the stem of the portobellos. Rinse mushrooms and pat dry.
Mix harissa, 1 1/2 tablespoons olive oil, cumin, and onion powder in a bowl. Brush each mushroom with the harissa mixture, making sure to cover the edges of the mushroom as well. Let mushroom marinade for 15 minutes.
While the mushrooms are marinating, prepare guacamole. Halve and pit the avocados and scoop out the flesh. Mash avocados and mix in chopped tomatoes, red onion, lemon (or lime) juice, salt, and cilantro. Set aside.
Rinse collard greens. Chop off the tough stems and set aside.
When the mushrooms are done marinating, heat 1 1/2 tablespoons of olive oil in a skillet or sauté pan over medium-high heat. Place the portobello mushrooms in the pan and cook for 3 minutes. Flip over and cook for another 2 to 3 minutes. Each side should be browned.
Turn off the heat and let the mushrooms rest for 2 to 3 minutes before slicing.
Take a collard green leaf and fill it with a few slices of portobello. Add guacamole, chopped tomatoes, cashew cream, and cilantro to your liking.