Sprinkle salt and pepper on steaks, refrigerate for one hour, remove to room temperature for 30 minutes prior to cooking.
Heat oven to 250 degrees F. Heat oil in heavy-bottomed pan on medium high to high, depending on your stove.
Sear steaks until browned, about 4 minutes on each side. Reduce heat to medium low, continue to turn steaks occasionally and baste with pan juices, about 4 additional minutes. Transfer steaks to oven-proof dish with sides. Pour remaining pan juices to a small bowl. Place 3 Tablespoons back into same pan.
On a medium high heat, saute` garlic 30 seconds then onions and saute` an additional 2 minutes. Add thyme and rosemary.
Add cognac and port, scraping any brown bits from the bottom of pan, continue cooking on medium-high hear for 1 minute. Add chicken stock and boil (on high heat) until reduced to about 1 1/2 cups, about 10 minutes.
Whisk-in mustard, and butter, lower heat to medium.
Return steaks to pan, cover and reheat steaks, adding any juices from the steaks back to pan. Serve with garlic mashed potatoes and a vegetable or a green salad.