In a small bowl, combine the Dijon mustard, tomato paste, and Worcestershire sauce. Set aside.
Brush the sides of beef with a few drops of fish sauce and season with black pepper to taste. Heat a large, heavy skillet over medium heat and heat 2 tablespoons of grapeseed oil until the oil is very hot. Cook the steaks evenly on both sides to the desired doneness and set aside to rest.
Remove the fat from the pan, leaving the fond in the pan. Add shallots to the pan with 2 tablespoons of butter and and sauté until softened, 2 minutes. Turn off the heat, remove skillet from stove, and carefully add the brandy. Return the pan to the stove and flambé. (Alternatively, let alcohol cook off on low heat.) Add mustard mixture and stock to the pan and bring to a boil and let simmer about 3 minutes. Remove from the heat and swirl in the remaining 2 tablespoons of butter. Add lemon juice to taste and sprinkle in the chopped chives. Add salt as needed.
Serve each steak on a plate with the sauce and pomme frites on the side (see recipe below).
In a heavy bottom pot, heat oil to 350° Fahrenheit.
Peel the potatoes and cut into ½ inch slices. When the oil has reached temperature, add the potatoes and stir so they do not stick together. Adjust heat to maintain temperature and cook for about 15 minutes until golden brown. Strain and place on a plated lined with a paper towel. Sprinkle salt immediately to finish.