Peel and chop the onions, melt them in a frying pan over low heat, with 10 g margarine and the oil 7 to 8 min, time to clean the mushrooms and mince them. Add the mushrooms with onions, then the lemon juice and 20 g of margarine, cover, let steam for 10 min shaking often the container, remove the lid, leave on very low heat if the mushrooms water is not evaporated, without letting go.
At the same time, in a saucepan, over low heat, mix 20 g margarine and flour. When the roux blanc is homogeneous, add the cream parve and sugar, reduce 7 to 8 min stirring very often. Salt and pepper. Out of the fire, mix the mustard.
Cut the meat slices into 2 cm wide strips, cut into 3 cm pieces. Return to skillet on high heat, 30 to 50 seconds per side in remaining margarine added as cooking progresses.
Add the meat to the contents of the sauté pan, top with the cream parve, stir in bringing to a simmer without boiling.
Serve very hot.
Serve with a Sancerre or Meursault wine.