Preheat oven to 375°F.
Sift flour with salt and set aside. In a heavy saucepan heat the water over medium. Add butter pieces and bring to a boil. Immediately add flour mixture all at once and stir with a wooden spoon. Remove mixture from heat and continue beating mixture until it comes away from the side of the pan. Return mixture to heat and stir slightly longer to dry the mixture. Remove mixture from heat once again. Add the eggs one at a time. Continue beating with the wooden spoon until mixture is shiny and falls from the spoon. Prepare a medium piping bag with a small plain tip. Fill bag with choux pastry. Leaving at least an inch between, pipe 16 rounds on to parchment covered baking sheets. Break egg into small bowl and beat. Brush choux lightly with egg wash. Bake until brown and hollow sounding about 15 minutes. Cool completely. Slice the tops of choux off. Save tops.
Combine mustard, pepper, rosemary, thyme and salt. Rub onto beef. In a very hot skillet sear the beef on each side for 1 to 2 minutes. Finish cooking 8 to 10 minutes in oven.
Heat butter on high in a medium saucepan. Add the shallots and sauté for 3 to 4 minutes until soft and golden. Add garlic, mushrooms and herbs. Cook over medium heat stirring frequently until mushrooms are dry and mixture is reduced by almost one half about 12 to 15 minutes. Adjust seasoning and set aside.
Preheat oven to 450°F.
Remove marrow from bones and set aside. In a large roasting pan roast veal bones in a little vegetable oil for 7 to 10 minutes until golden brown. Add vegetables and herbs. Return to oven and reduce temperature to 375 degrees F. Roast until vegetables are browned 25 to 30 minutes. Add tomato paste and wine. Scrape the bottom of the roasting pan and transfer all contents to a large stockpot. Add water and bring to a boil over high heat.
Once stock begins to boil, reduce heat to very low to sustain a slow boil. Simmer uncovered on low heat for at least 3 to 4 hours or for as long as 8 hours. Let stock reduce by half. Strain and discard bones and vegetables. Cool and skim off fat.
In a small saucepan cook olive oil and shallots for 3 to 4 minutes. Deglaze with red wine and reduce down to a third. Add the veal stock and reduce down to half. Add marrow and simmer 5 to 10 minutes until marrow has melted. Adjust seasoning.
Spoon a little of the cooled duxelles into the bottom of each choux. Dice the cooked meat. Spoon over top of duxelles. Put some sauce in a squeeze bottle. Drizzle a little sauce over each choux. Cover with its top. Drizzle with a little more sauce.