Place a baking tray in the preheated oven (electric cooker: 80 °C/ circulating air: not suitable/ gas: not suitable). Mix mustard, 2 tablespoons of oil and honey. Wash tarragon, dab dry, put something aside for garnishing.
Finely chop the rest, stir into the mustard cream and season with pepper. Wash, clean and halve the tomatoes. Halve the garlic. Wash the meat and dab dry. Season with salt and pepper. Heat 2 tablespoons of oil in a pan.
Sear the beef fillet on all sides. Add tomatoes and garlic, also fry. Put meat, tomatoes and garlic on the preheated baking tray. Spread the fillet with the mustard cream. Season tomatoes with salt.
Cook in the preheated oven for about 4 hours. Do not open the oven door during this time. Peel, wash and cut the potatoes into pieces. Cook in boiling salted water for about 20 minutes.
Warm milk and butter. Drain the potatoes and let them evaporate briefly. Mash with crème fraîche and milk mixture to a puree. Season with salt and nutmeg. Serve the fillet of beef with tarragon set aside.
Serve immediately together with the puree.