In a small bowl, combine the rosemary, garlic, salt, pepper and zest. Rub filets with half tablespoon olive oil, then rub herb mixture onto filets. In a large sauté pan over high heat, heat the remaining 1 tablespoon of olive oil until hot. Add the filets and cook to your liking, about 3 minutes per side for rare. Remove filets from pan, transfer to a warm plate. Add shallots and red pepper to pan; reduce heat to medium and cook for 2 minutes, stirring occasionally. Add chicken stock and lemon juice to the pan and cook until sauce reduces by one third. Stir in butter; cook for another 2 minutes. Spoon sauce over filets and serve.