Using chef’s knife trim all fat from the tenderloin. Cut tenderloin into 1 inch medallions and sprinkle with rosemary and half each of the salt and pepper.
In large non-stick skillet heat half of the oil over medium high heat and brown pork in batches if necessary.
Remove to plate. Return skillet to heat and add remaining oil. Cook onions, stirring for about 8 minutes or until beginning to turn golden. Reduce heat to medium and cook, stirring for about 10 minutes or until very soft and golden. Add red pepper and garlic and cook for 2 minutes.
Add stock and Worcestershire sauce and remaining salt and pepper. Bring to boil and boil for 5 minutes or until reduced by half. Return pork to skillet and turn to coat in sauce for about 2 minutes or until a hint of pink remains in pork.