Boil the rice in plenty of water until just cooked, then drain.
Meanwhile, heat a dash of oil in a large frying pan, and add the onion and mushrooms. Cook for a few minutes over a medium heat until slightly softened, then add the garlic, smoked paprika, cumin and tomato puree. Cook for a few more minutes, stirring regularly.
Add the black beans and the cooked rice, and season generously. Mix well to combine.
Take one of the large flour tortillas, and add about 1/4 of the rice mixture to the centre. Top with 1/4 of the grated cheese. Fold two sides of the tortilla inwards, then rotate through 90° and fold in the other two sides to fully wrap the burrito (see the blog post for more detailed burrito-folding tips!).
Repeat with the remaining tortillas, rice, and cheese, and place the burritos seam-side down on a baking tray.
In a small bowl, combine the finely chopped jalapeños, coriander, and sour cream. Season with a pinch of salt and pepper, and mix well.
Divide the sour cream sauce over the 4 burritos, and spread it around a little. Bake at 190°C (Gas Mark 5 / 375°F) for around 15-20 minutes, or until crisped up to your liking. Serve with your choice of toppings.