Chop onion and take it in a large pan over medium heat. Saute it with 3 tbsp of olive oil for 5 minutes. Wash, drain and cut mushrooms into small cubes. Place mushrooms into the pan and stir. Cover lid and cook until it absorbs all of its juice, stirring occasionally. Add boiling water into the pan. Add black pepper, rice, and salt to the pan, stir. Turn the heat down to low and cover the lid. Simmer until the rice absorbs all of the water.