Brush mushrooms with olive oil and place on a baking sheet. Bake for 15 minutes, flipping once, until mushrooms are tender.
In a medium saucepan over medium heat, add the cannellini beans, garlic powder, paprika, cayenne pepper, cumin, coriander, and vegetable broth. Bring to a simmer, reduce heat and cook, stirring occasionally, until the beans are creamy, about 8 minutes.
Stir in the harissa sauce, and season with salt and pepper to taste.
Serve mushrooms with mashed cannellini beans and harissa sauce.
Interesting Facts
Harissa is a North African chili pepper paste.
Portobello mushrooms are actually the same species as white button mushrooms, just at a more mature stage.