Add 1 Tbsp (15 ml) oil to a large skillet and heat over medium-high heat. Once hot, add red onion and sauté, stirring occasionally, until beginning to brown and soften — about 3-4 minutes. Add the bell pepper and cook until tender with a little bite — about 4-5 minutes. Transfer the cooked peppers and onions to a bowl or plate and set aside.
To the now empty skillet, add 1 Tbsp (15 ml) oil and as many sliced mushrooms as will fit in a single layer in the pan. Allow to cook on one side for 2-4 minutes or until lightly browned, then flip and cook on the other side for another 2-4 minutes. If you have any remaining mushrooms, remove the cooked mushrooms from the skillet and cook remaining sliced mushrooms in 1 Tbsp (15 ml) oil.
Once all the mushrooms are cooked, turn the heat to low and return all the mushrooms, onions, and bell peppers to the skillet. Add the oregano, garlic powder, and salt. Toss to coat and heat through, then remove from heat.
If using canned refried beans, heat them in a saucepan or in the microwave until warmed through.
Warm tortillas by wrapping in a damp paper towel (or kitchen towel) and microwaving for 30 seconds - 1 minute. If using fresh or homemade tortillas, you can warm them one at a time in a cast iron skillet over high heat (or on a hot grill) for 10-20 seconds on each side (or until just warmed), then wrap in a towel to keep warm until serving.
To serve, top warmed tortillas with refried beans, the portobello fajita filling, chipotle pecan pesto, and cilantro (optional).
Any leftover pesto can be used in bowls, salads, or as a dip for veggies and crackers. Pesto will keep stored in a sealed container in the refrigerator for 4-5 days.
Best when fresh. Leftover fajita filling will keep in a sealed container in the refrigerator for 2-3 days. Reheat fajita filling in a skillet over medium heat until warm. Not freezer friendly.
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Notes
*Recipe as written makes ~2 ¾ cups of filling. *Nutrition information is a rough estimate calculated with the lesser amount of avocado oil and a full batch of Chipotle Pecan Pesto.