Season meat with salt and pepper, then dip in flour and shake off excess.
In heavy 10 - 12 inch skillet, melt butter with the oil over moderate heat.
When foam subsides, add the meat, 3 or 4 at a time, and brown for about 3 minutes on each side.
After browning, transfer to a plate.
If adding mushrooms, saute at this point and remove from pan, then place on platter with the meat.
Pour off most of the fat from the skillet, leaving a thin film on the bottom.
Add Marsala and 1/2 cup of stock and boil the liquid briskly over high heat for 1 or 2 minutes.
Scrape in any browned fragments clinging to the bottom and sides of pan.
Return meat and mushrooms to skillet, cover pan and simmer over low heat for 10 to 15 minutes, basting now and then with pan juices.
To serve, transfer meat to a heated platter.
Add 1/2 cup of stock to the sauce remaining in the skillet and boil briskly, scraping in the browned bits clinging to the bottom and sides of the pan.
When the sauce has reduced considerably, and has the consistency of a syrupy glaze, taste it for seasoning.
Remove the pan from the heat, stir in the soft butter and pour the sauce over the meat and mushrooms.