Rub roast beef with oil, salt and pepper. Place on the fat pan of the oven and bake in the preheated oven (electric oven 200 ° C/ gas: level 3) for approx. 45 minutes. Wash parsley, dab dry and chop finely.
Peel the garlic and press it through the garlic press or chop it finely. Mix 50 g butter, parsley, garlic, salt and pepper. Cut the baguette into slices and spread with the butter.
Peel and finely chop the onion. Heat the remaining butter in a pot and fry the onions until transparent. Deglaze with the cream and let it boil down a little. Add mustard and capers and stir in the sauce thickener.
Season to taste with salt and pepper. Remove the roast beef and let it rest for a while. Bake bread in the hot oven until golden brown. Cut the roast beef and arrange it on a plate with the bread. Garnish with parsley and radishes.
Extra sauce is fine. Serve with a salad of lollo bianco and radishes.