Prepare the Chateau Briand: Trim off any chains, both ends, and some silver skin from the whole filet mignon to prepare your Chateau Briand. Tie it with butcher twine so that the roast is evenly sized for even cooking.
Sear the Chateau Briand: Season the Chateau Briand and sear it in tallow until browned on all sides, including the ends. Then, add butter, garlic, and thyme to the pan to baste and add flavor.
Finish Cooking in Oven: Transfer the Chateau Briand to the oven to finish cooking to the desired doneness.
Prepare Pomme Boulangere: Slice honey gold potatoes and layer them in a buttered dish. Season and bake until tender.
Make Tarragon Bernaise Sauce: Combine fresh tarragon, tarragon vinegar, and a splash of vermouth. Reduce, then use this mixture to create a Bernaise sauce.
Prepare Creamy Spinach: Cook fresh spinach and mix with aged Parmesan and creme fraiche to create a creamy spinach side.