First make sauce, it requires much more attention than the meat. Melt in a saucepan the butter and add flour. Will heat for a few minutes, stirring with a spatula.

For the sauce I used butter "Ecomilk".

When the creamy starchy mass will be homogeneous, at once pour in cold milk and quickly mix with a whisk. So not formed lumps and the sauce will not need to strain. Prolonged heating. When the sauce thickens, add a pinch of nutmeg, salt, sour cream and tomato paste. Stir well with a whisk our sauce. It will turn a beautiful pink color. Remove the sauce from the heat and cover it to the surface film was formed.

Now let's get to the meat. Cut beef tenderloin into thin slices.

Cut onion into half rings and fry in vegetable oil (3 tbsp) until Golden brown.

A third of onions sent to the sauce, the rest leave in the pan. In a pan add a little vegetable oil (1 tbsp) and on high heat fry the tenderloin on the onions. Once the tenderloin was roasted on one side, gently turn it and fry for another 2-3 minutes. Sprinkle with salt and pepper. The main thing - not prigotovit meat, it is juicy and delicious.

Omit the meat to the sauce and warmed in a water bath or on very low heat for 3-4 minutes. A La carte is served in small cups, sprinkled with parsley (it can be cut with scissors directly into the form).