This is a classic, casual French dish, the kind of meal you would be served in someone's home or in a bistro for lunch. I've always eaten it with buttered white rice, and wouldn't want to mess with perfection. Below is a basic, everyday version that is really quite healthy, followed by two variations: one decadent and one vegetarian. Choose the one that works for you!
For two people, you will need:
2 chicken cutlets, about 1/3 pound each, lightly salted. Chicken cutlets usually refer to pieces of boneless skinless chicken breast that have been butterflied, or cut in half horizontally, to create thinner and more quickly and evenly cooking pieces of meat. (If you can't find decent cutlets in your grocery store, just buy boneless skinless breasts and make your own.) To tenderize and even out the thickness, lay the chicken cutlets on a cutting board, cover with a piece of plastic wrap, and lightly pound with a meat tenderizer or mallet. I don't have either, so I used the bottom of a glass mustard jar.
1/3 pound small crimini or white mushrooms, gently wiped clean with a damp paper towel. (Don't wash - you want them to be as dry as possible so they caramelize when you saute them.) Trim the ends of the stems and slice mushrooms in half. If they are large mushrooms, cut into thick slices.
1 small yellow onion, cut in half lengthwise, peeled, and then thinly sliced crosswise.
1 1/2 cups chicken stock
1/4 cup white wine (or another 1/4 cup chicken stock)
2 tablespoons flour, put into a jar with 1/3 cup water. Put the lid on and shake vigorously to make a lump-free slurry, which you will use to thicken the sauce.
2 teaspoons whole grain mustard. I like Plochman's Natural Stone Ground or Maille Old Style. You can use regular Dijon, and the result will be smoother, spicier, but less nutty and sweet.
2 tablespoons fresh tarragon, finely chopped.You can also use fresh thyme, but there's something about the grassy, anise flavor of tarragon that is really special here. You can use dried if you must, but it won't have the same brightness of flavor.
Note: Use a wide saute pan that has a stainless steel interior, rather than teflon or other nonstick surface. This will allow you to get good caramelization on the mushrooms and chicken, which makes for a richer sauce.
In a stainless steel pan, heat a tablespoon of olive oil over medium heat. When the oil starts to shimmer, add the mushrooms, and push around in the pan until well coated with oil. Sprinkle lightly with salt. Cook until dark golden brown on each side. Remove mushrooms to a separate dish, and return pan to the stove at medium heat.
Add a touch more olive oil and the onions. Sprinkle lightly with salt and cook, stirring frequently, until soft, translucent, and just starting to brown. Remove to a separate dish and return pan to the stove.
Add a touch more oil, raise the temperature slightly, and add the chicken cutlets. Let the chicken sit in the pan for about 30 seconds, then lift each cutlet up and replace in the pan. This will prevent sticking. Fry until the chicken is cooked through and you get a beautiful, dark reddish brown color, 2-3 minutes per side. Remove the chicken to a separate dish and return pan to the stove, lowering the heat to medium.
Now you want to turn those delicious brown pan drippings into a rich sauce. Deglaze the pan by adding the white wine - if you're not using wine, then 1/4 cup chicken stock - and loosen the brown bits from the bottom of the pan with a wooden spoon or scraper. When the pan drippings have dissolved into the wine, add the stock. Bring to a simmer, then slowly add the flour slurry, whisking as you pour, until the sauce has thickened to the consistency of a thin gravy. You may not need all the flour. Simmer for about three minutes, whisking constantly, until the raw flour is cooked and the alcohol in the wine has evaporated. If it seems too thin, add more flour slurry. If it seems too thick, add more chicken stock. The sauce will reduce to about 3/4 cup to 1 cup.
Whisk in the mustard and a healthy dose of black pepper. Return the onions and mushrooms to the pan and stir in the chopped tarragon. Return the chicken to the pan, remove from the heat, and let sit for a minute to meld the flavors. Serve immediately with hot, buttered white rice or noodles.
Rich, Gluten-Free Variation: Deglaze the pan with 1/4 cup white wine. Add 1 1/2 cups heavy cream. Simmer for about 3 minutes over low heat, whisking regularly, until reduced by half. Stir in 2 tablespoons of butter and the mustard, then the onions, mushrooms, tarragon, and chicken.
Vegetarian Variation: Double the mushrooms. Use white wine and vegetable stock to make the flour-thickened sauce, or follow the Rich Variation above.