Crush the whole peppercorns by placing them between two tea towels until they are coarsely ground.

In a bowl, mix the crushed peppercorns with the finely chopped rosemary, minced garlic, chopped shallots, mushrooms, and a generous amount of heavy whipping cream. Add a pinch of salt and whisk until the sauce is well combined.

Prepare the chicken by spatchcocking it; cut along the breastbone so that it lays flat while keeping the backbone intact.

Season the underside of the chicken with salt, then rub olive oil on the skin side and season that side as well.

Place the chicken in a preheated oven set to 375°F and roast for about 15 minutes, or until it is lightly golden.

Pour the prepared cream mixture over the chicken and add a splash of dry sherry cooking wine.

Return the chicken to the oven, increase the temperature to 400°F, and continue cooking until the chicken is fully cooked, basting it twice during the cooking process. (If using bone-in, skin-on thighs, bake them 20 to 30 more minutes. Boneless thighs cook faster; give them only 5 to 10 more minutes. Check that the thickest part of the meat reads at least 165°F for safety. Thighs are more tender and juicy at 175° to 185°F, since the extra heat breaks down connective tissue, so many cooks let them go a bit past 165°F.)

When it's done, remove the chicken from the oven and let it rest for 10 minutes before serving.