Wash the peppers, and clean the dirt from the portobello mushroom. Slice the peppers and slice the onion into thick strips. Slice the mushroom (if not using a grill pan or basket, chop into larger pieces). Mince the garlic (separate and set aside 1 clove minced garlic).
In a large bowl, mix the vegetables with the 2 cloves minced garlic, 2 tablespoons olive oil, 2 tablespoons fajita seasoning, and ½ teaspoon kosher salt (decrease the salt if the fajita seasoning contains salt).
Drain and rinse the beans, and place them on a square of aluminum foil. Mix the beans with 1 tablespoon olive oil, the other clove diced garlic, 1 tablespoon fajita seasoning, and ½ teaspoon kosher salt. Fold up the sides of the foil to form a square packet.
Place the vegetables in a grill pan, then place it on the grill and cook for about 10 minutes, until tender, turning occasionally. (If not using a grill pan, place larger vegetable pieces directly onto the grill.)
When the vegetables are nearly done, place the bean packet on the grill to warm the beans, 3 to 5 minutes. Remove the vegetables and bean packet from the grill, and serve with tortillas and garnish with sour cream.