1½ pounds beef tenderloin, trimmed of fat and cut into 2x½x½-inch strips
½ teaspoon salt or to taste
½ teaspoon black pepper or to taste
1 large yellow onion, peeled and thinly sliced
¼ cup (½ stick) butter
2 tablespoons flour
1 cup beef broth
2 teaspoons Dijon mustard
1/3 cup sour cream (at room temperature)

Spread tenderloin on wax-paper-lined counter, sprinkle with salt and pepper, and toss well to mix. Cover with onion slices and toss well again. Cover loosely with wax paper and let marinate 1 hour.

Melt 2 tablespoons butter in large heavy skillet over moderately low heat and blend in flour. Add broth and cook, stirring constantly, until thickened and smooth---about 3 minutes; blend in mustard and set off heat.

In a second large heavy skillet, melt remaining 2 tablespoons butter over moderately high heat. Add tenderloin and onion and brown well—8 to 10 minutes.

Add browned tenderloin—but not onion---to first skillet, set over low heat, and simmer, stirring occasionally, 10 minutes.

Off heat, smooth in sour cream. Taste for salt and pepper, adjust as needed, and serve with wide egg noodles.