– Step 1: Preheat oven to 400F. Heat a large skillet. Add 1 tsp olive oil, garlic, and the onion. Cook, stirring often until it softens, about 4 minutes. Add the mushrooms and jalapeno and continue to cook until mushrooms soften. – Step 2: Add black beans, corn, tomatoes, and spices. Cook for a few more minutes to allow most of the tomato liquid to cook off. Step 3: Meanwhile, prepare the peppers by cutting them in half through the stem end (vertically), removing the seeds and membranes, and rinsing the insides to make sure all the seeds are out.