1. Cut the tenderloin into 1/2 inch (about 1cm) thick slices. Finely chop the garlic.
2. Place the pork pieces in a medium bowl and season with chili powder, salt and pepper. Set aside.
3. Cut the mushrooms into bite size pieces. Slice the onion into quarter rings.
4. In a large skillet, heat 2 tablespoons oil. Brown pork in oil for about 3 minutes on each side. Remove from the skillet, place on a separate plate and set aside.
5. Add 1 tablespoon oil to the same skillet. Saute the onion, garlic and mushrooms for about 5 minutes. Stir in the flour until well blended. Gradually stir in the chicken broth (or water). Bring to a boil. Cook stirring for about 2 minutes, or until thickened.
6. Add the pork medallions and rosemary to the mixture. Reduce heat, cover and simmer for 30 minutes, or until the meat is tender and its juices run clear.