Slice the stem off of the peppers so that they will stand steadily when flipped upside down. Slice the peppers into quarters from the bottom to the top around the core and seeds. Slice each quarter into 1-inch (2 ½ cm) thick slices. Transfer to a large bowl.
Add ½ teaspoon salt, ¼ teaspoon cumin, ½ teaspoon smoked paprika, 1 teaspoon chili powder, and ½ teaspoon red pepper flakes. Toss to coat the peppers evenly in the spices and set aside.
Remove the stems from the portobello mushrooms and slice into 1-inch-wide (2 cm) pieces, then transfer a large bowl.
Add the remaining ½ teaspoon salt, ½ teaspoon chili powder, ½ teaspoon smoked paprika, ¼ teaspoon cumin, and ½ teaspoon red pepper flakes. Toss to coat the mushrooms evenly in the spices and set aside.
Thinly slice the onion.
Heat oil in a large cast-iron pan over medium-high heat. Add the onion and cook, stirring occasionally, until translucent, about 3 minutes. Add the garlic and bell peppers, and cook for about 5 more minutes, stirring constantly until peppers are tender. Remove the pepper and onion mixture from the pan and set aside.
Add the mushrooms to the skillet and cook until the mushrooms are tender, stirring constantly, about 5 minutes
Divide the brown rice between 4 food storage containers. Top each container with pico de gallo, black beans, corn, cheddar cheese, the cooked bell peppers and onions, and the sliced portobello. Garnish with an avocado slice, a lime wedge, and cilantro.