Preheat an oven to 350°F. Hack the chicken breasts in half crosswise. Rinse the chicken pieces and pat dry with paper towels. Sprinkle with the 1 teaspoon salt and 1/4 teaspoon pepper.

Heat 2 tablespoons of the chicken fat or oil in a large ovenproof sauté pan with a lid over medium-high heat. Add the mushrooms and onions, reduce the heat to medium, and sauté until the onions are very soft gold but not crisp and the mushrooms have shrunk, about 15 minutes.

Meanwhile, in another large sauté pan, heat the remaining 1 tbsp fat over high heat. Add the chicken, skin side down, and sauté, turning once, until well browned, about 5 minutes on each side. Remove the chicken to a plate. Pour off all the fat from the pan. Return the pan to high heat and pour in the stock. Bring to a boil.

When the mushrooms are done, scrape them out of the pan into a bowl. Add the kasha to the pan and stir and toss over medium-high heat until the grains smell toasty, 2 to 3 minutes.

Stir the boiling stock into the kasha. Add the mushrooms, folding them in completely. Add a touch more salt and white pepper. Nestle the chicken pieces in the kasha, skin side up. Cover and bake for 30 minutes. Uncover, stir in the parsley, and bake until all the liquid is absorbed, the kasha is tender, and the chicken is cooked through, about 5 minutes longer.

Serve directly from the pan, spooning kasha and chicken together.