1½ tbspolive oil4200gm pieces of beef tenderloin fillet150 mlred wine200 mlbeef stockGratin dauphinoise500 gmfloury potatoes, such as sebago750 ml (3 cups)pouring cream2 tspthyme leaves, plus extra to serve1garlic clove, finely choppedTo serve:coarsely chopped flat-leaf parsleyRoquefort sauce200 mlpouring cream100 gmRoquefort, coarsely crumbled
For gratin dauphinoise, preheat oven to 200C. Thinly slice potatoes on a mandolin and layer in a buttered 1.5-litre baking dish, seasoning generously to taste as you go. Bring cream, thyme and garlic to the simmer in a saucepan over medium-high heat, then pour onto potato to cover (depending on the size of your baking dish, you may not need all the cream). Place baking dish on an oven tray and bake on the lowest shelf of the oven until bubbling and golden and potato is tender when pierced with a skewer (50 minutes to 1 hour). Scatter with parsley and extra thyme just before serving.
Heat oil in a large frying pan over medium-high heat, add beef, season to taste and cook, turning once, until browned (2-3 minutes each side). Transfer to oven and roast until cooked to your liking (4-5 minutes for medium-rare), then transfer beef to a warm plate and rest for 5 minutes. Deglaze pan with wine and simmer over medium-high heat until reduced by half (2-3 minutes). Add stock, simmer until reduced to a glaze (2-3 minutes) and season to taste.
Meanwhile, for Roquefort sauce, combine cream and half the Roquefort in a small saucepan over low heat and stir occasionally until melted and combined (2-3 minutes). Season to taste and keep warm.
Drizzle glaze over beef and serve with gratin dauphinoise. At the table, spoon Roquefort sauce over beef and scatter with remaining Roquefort.