Preheat oven to 400 degrees and prepare a baking sheet with foil or use a non-stick baking sheet. Thoroughly rinse produce and pat dry. Peel onion. Slice half into thin strips and finely dice the other half. Use a spoon to clean out black undersides (gills) of portobello mushrooms. Cut mushrooms into ¼" strips. Core and slice red bell pepper into ¼" strips. Thinly slice green onion on a bias (angle). Zest and halve lime.

Place poblano pepper on baking sheet and roast until skin blisters and becomes dark, 13-15 minutes. Remove from oven and pull up corners of foil to enclose pepper, allowing it to steam inside foil. Reduce oven temperature to 200 degrees. Once cool enough to handle, use your hands to carefully remove stem, skin, and seeds from pepper. Finely dice pepper and set aside. Roasting and peeling the poblano mellows its flavor, softens its texture, and gives it a smoky flavor.

While pepper is roasting, warm a medium pan over high heat. Add 1 tsp. olive oil and sliced red onion. Cook for 1 minute, or until aromatic. Add sliced red bell pepper and portobello mushrooms. Cook for another 4-5 minutes, or until lightly browned, add 1 Tbsp. chili powder and 1 Tbsp. water. Reduce heat to medium and cook for another 3-4 minutes, or until vegetables caramelize. Set aside.

In a medium pot over medium heat, add 1 tsp. olive oil and red onion and cook until aromatic, about 2-3 minutes. Add black beans (including liquid from can), 2 Tbsp. water, remaining chili powder, juice of half the lime (to taste), and half the diced roasted poblano pepper (reserve remaining for topping tacos). Mash beans in pan with a potato masher or large fork. If desired, add 1 Tbsp. of water at a time to adjust consistency of beans to your liking. Season with a pinch of salt and pepper.

Replace foil on the same baking sheet used to roast the pepper and evenly arrange tortillas on it. Bake for about 3-5 minutes in the 200 degree oven, or until soft and warm. In a small mixing bowl, combine the zest and juice from half the lime (to taste) with the sour cream. Season with a pinch of salt and pepper.

Place 3 warmed tortillas on a plate. Spread a layer of black beans on the tortillas. Nestle cooked veggies into the tortillas. Top with remaining diced poblano pepper. Drizzle crema over tacos, or serve on the side. Garnish with green onions.