Broil the veggies: Preheat a broiler. Thinly slice the bell peppers and thinly slice the onion. Remove the stems and slice the portobello mushrooms. Place them in a bowl and toss with 2 tablespoons oil and 1 teaspoon kosher salt. Spread the veggies on a parchment lined sheet pan. Place in the broiler and broil 8 to 10 minutes, stirring halfway through, until softened and blackened on some edges. Then go right to Step 3.
Cook the beans: Meanwhile in a skillet, whisk together the ingredients for the Homemade Fajita Sauce. Drain and rinse the pinto beans and add them to the skillet. Heat over medium heat and cook until warmed through and the sauce is thickened, about 6 to 8 minutes. Taste and stir in another ¼ teaspoon kosher salt.
Saute the veggies: Once the veggies are done in the broiler, finish them on the stove by heating the remaining 2 tablespoons oil in a cast iron skillet or large non-stick skillet over medium high heat. Add the broiled veggies, smashed garlic cloves, cumin, chili powder, garlic powder, smoked paprika, and the remaining ½ teaspoon kosher salt. Cook for 5 minutes, stirring occasionally, until the veggies are tender. Remove and discard the garlic cloves before serving.
Warm the tortillas: Warm and char the tortillas by placing them on an open gas flame on medium for a few seconds per side, flipping with tongs, until they are slightly blackened and warm. (See How to Warm Tortillas.)
Serve: Place bowls of fajita veggies and pinto beans on the table, with tortillas, sour cream, pico de gallo, guacamole, and fresh cilantro. Let each person add their desired toppings and serve.
