Heat a nonstick pan with olive oil. Preheat oven to 350°F.
Salt and pepper the fillets. Coat each fillet with Dijon mustard and then crust with shredded potato, pressing down to form smooth the potato over the top. If you grated the potatoes ahead of time, you can keep them in cold water until ready to use.
Add fillets to frying pan, potato side down and sear until golden brown and crispy, about 3 minutes. Remove to a baking pan, potato side up, and finish by baking in oven, about 6 to 7 minutes, depending on thickness of fillet.
Prepare the sauce. Heat the butter in a small frying pan. Add shallots, peppercorns, garlic, and thyme, and sauté until shallots are tender. Add the Dijon mustard and flour and whisk until smooth. Whisk in the cream. Season to taste with salt and pepper.