Place the steaks in a non-aluminum container and top with pureed onion. Let stand for 15 minutes, then remove the onion, turn the steaks over, cover with onion again and let stand a further 15 minutes. Scrape off the onion and discard it. Pat the steaks gently on both sides with a paper towel to dry. Salt and pepper the steaks to taste.
Melt the 2 tablespoons of butter in a pan and sauté the steaks to desired doneness. Remove from pan and keep warm.
To make the sauce, melt 3 tablespoons butter in the same pan and sauté the onions and garlic until translucent and softened. Carefully add the cognac and cook for a minute or two. Add the red wine carefully and cook over high heat until the sauce starts to thicken. Season to taste. Just before serving, add the remaining tablespoon of cold cubed butter and stir to combine to finish the sauce. Pour over steaks and serve immediately. Garnish with watercress, parsley or chives if you like.