For the beef stew:
Season meat all over with salt and pepper. Heat a large Dutch Oven over medium heat until hot for no longer than 5 minutes. Add enough oil to cover the bottom of the pot. Add the beef and sear until browned on all sides. Remove from the pot and set aside.
Add the celery, carrots and shallot to the pot, and cook until slightly golden brown in color, about 5 minutes. Add the thyme, rosemary, tomato paste, garlic, peppercorns and bay leaf to the pot and cook for about 2-3 minutes. Add the warm beef stock to the pot and use a silicone whisk to scrape all of the concentrated flavor (fond) off the bottom of the pot. Add the beef back to the pot and bring to a simmer over low heat. Cover with a lid slightly cracked, and simmer over low heat until the beef is tender, about 2 -1/2 to 3 hours.
Add the salt and cornstarch mixture to the pot and stir until sauce is slightly thickened. Remove from heat and allow to rest 30-40 minutes to allow the meat to relax and become more tender.
For the fondant potatoes:
Peel and wash the potatoes and cut them horizontally into 2 1/2-3 inch cylinders. Dry the potatoes thoroughly with a paper towel.
Heat a large pot or Dutch oven over medium-high heat for no longer than 5 minutes, and add about 3 tablespoons of oil to the pot. When the oil is hot, sear the potatoes on 1 side until golden brown, about 5 minutes. Flip the potatoes so the seared side is facing up and add the salt, garlic and shallot followed by the unsalted butter and the thyme sprigs.
Reduce heat to low and allow the potatoes to slowly cook in the butter, turning them completely over every 20 minutes until a knife can be easily Inserted into the center, about 2 1-2 hours.
For the creamed leeks:
Remove any wilted and chewy outer leaves from the leeks, and slice horizontally into perfect 1/3-inch circles. Drop the leeks into a bowl of cold water and mix gently with your hands to remove all of the dirt. Remove leeks from the water and dry thoroughly with paper towels.
Pour heavy cream into a small saucepan, braiser or Dutch oven and bring to a simmer over low heat. Add the leeks and cook until the leeks are tender and creamy, about 20 minutes. Turn off the heat and add the mustard. Season to taste with salt and pepper.
To serve, place some of the creamed leeks on a plate, top with 2-3 of the potatoes and some of the braised beef.