- 1/4 cup plus 1 teaspoon kosher salt
- 3 tablespoons sugar
- 1 quart cool water
- 4 boneless, skinless 1-inch-thick chicken breast halves (about 2 1/4 pounds)
- 4 tablespoons (1/2 stick) unsalted butter
- 3 tablespoons olive oil
- 3/4 cup sliced shallots
- 1 tablespoon minced garlic
- 3/4 cup all-purpose flour
- 2 teaspoons freshly cracked black pepper
- 1/2 teaspoon granulated garlic
- 1/2 cup dry white wine
- 1/2 cup Dijon mustard
- 2 tablespoons whole-grain mustard
- 1/2 cup chicken stock
- 3/4 cup heavy cream
- 4 cups loosely packed baby spinach, shredded
- 8 slices Emmenthaler or Swiss cheese (about 1/4 pound)
In a resealable 1-gallon plastic bag, dissolve the 1/4 cup kosher salt and sugar in the water. Add the chicken, seal tightly, and refrigerate for 1 hour or up to 8 hours.
Preheat the oven to 275 F.
In a large oven-safe skillet over medium-high heat, melt 1 tablespoon of the butter in 1 tablespoon of the oil. Add the shallots and cook until golden, stirring occasionally, about 3 minutes. Add the minced garlic and cook for 30 seconds more. Remove from the pan. Do not rinse the pan.
Remove the chicken from the brine, rinse, and pat dry. Slice the breasts in half horizontally, so that each piece is about 1/2-inch thick. Trim any fat. Season the flour with the remaining 1 teaspoon salt, pepper, and granulated garlic and combine well. Dredge the chicken breasts in the seasoned flour.
Combine the remaining 2 tablespoons oil and 3 tablespoons butter in the skillet and turn the heat to medium-high. Working in batches, add the chicken (do not crowd) and cook for 3-5 minutes, until browned on one side. Turn the chicken over and cook for 2-3 minutes to brown the other side. As the chicken finishes cooking, place it on a baking sheet in the oven to keep warm. Repeat with the rest of the chicken.
Pour the wine into the pan and cook until the pan juices are reduced to 2 tablespoons, about 3 minutes, scraping the browned bits off the bottom of the pan. Stir in the mustards and stock until smooth. Stir in the cream. Bring the sauce to a simmer, stirring frequently. Return the chicken breasts to the pan, turning to coat them evenly in the sauce and incorporate the coating into the sauce (this helps to thicken the sauce).
Increase the oven temperature to a low broil if you have it and make sure there’s an oven rack in the top third of your oven, or to 500 F if not. Evenly sprinkle the spinach on top of the chicken, evenly distribute the shallots and garlic on top of the spinach, and top with the cheese. Broil or bake in the oven until the cheese is melted and just starts to brown in some spots. Remove the pan from the oven and let it rest on a heatproof surface for 5 minutes before serving.