Heat the oil on medium heat in a large frying pan. Sauté onions for 4 minutes or until they start to turn translucent. Add mushrooms and cook for about 7 minutes or until wilted. Stir well. Drain any extra liquid.
Add chili powder and tomato paste to the pan. Stir and cook for another minute or two.
Divide the mixture into four portions and place lengthwise on the tortillas. Place cheese on top. Roll up and place in a casserole dish.
Bake in the oven for about 10 to 15 minutes or until cheese is melted.
Spoon the guacamole and salsa on top of the burritos and serve.