1. Cut the tenderloin into 1/3 inch thick (a bit less than 1cm thick) slices.
2. Get a chopping board and put a meat slice between two layers of food wrap (or inside a plastic sandwich bag). Using a meat hammer, bash each slice thinly (both sides), or pound with a mallet.
3. Season teach slice with salt and pepper (you should get about 10 slices).
4. Heat 2 tablespoons olive oil in a medium skillet over medium high heat. Saute onion for 3 minutes and then add the mushrooms. Cook for another 7 minutes.
5. Stir in the sour cream and season the mixture with salt and pepper. Turn off the heat.
6. Place 1 tablespoon of mushrooms on each meat slice. Roll the slices tightly (the mushrooms should be inside the roll). If you need, you can fix the rolls with toothpicks or strings.
7. Heat 2 tablespoons olive oil in the same or different skillet. Brown the rolls on all sides. Add the water, bring to boil, reduce the heat and simmer for 30 minutes, or until tender. Let rest for 5 minutes and serve.