Black beans are not usually added to this popular Mexican rice dish, but their addition makes it into a one-pot meal. Use whatever beans you have on hand, or leave them out if you want to serve this as a side dish.
Add the poblano pepper, green chiles, cilantro, and spinach to a blender and puree. Add some of the vegetable stock, as needed, to achieve a smooth consistency. Add the mixture to a medium saucepan with the remaining vegetable stock. Add the brown rice and bring to a boil over high heat. Reduce the heat to medium and cook, covered, until the rice is tender, 45 to 50 minutes.
Place the onion in a large saucepan and sauté over medium heat for 7 to 8 minutes. Add water 1 to 2 tablespoons at a time to keep the onion from sticking to the pan. Add the cumin, jalapeño pepper, and black beans and cook for 5 minutes longer. Fold in the cooked rice and lime zest. Season with salt.