Preheat the oven to 375°F. Remove the giblets and pat the chicken dry. Using kitchen shears, cut along both sides of the backbone to remove it. Flip the chicken over and press firmly on the breastbone to flatten it.
Pat the chicken dry again. Rub both sides generously with olive oil, salt, pepper, and Creole Trinity & Herb seasoning. Place skin-side up in a large oven-safe braiser or roasting pan.
Roast uncovered for 20 minutes.
Meanwhile, combine the cream, shallots, Dijon, sherry, Creole Trinity & Herb, black pepper, salt, and rosemary in a saucepan. Bring to a gentle simmer over medium-low heat, stirring. Do not boil.
Remove the chicken from the oven and increase the temperature to 400°F. Pour the warm sauce around the chicken, spooning some over the top.
Roast for 20 minutes, then baste the chicken with the sauce. If the sauce has thickened too much, stir in a splash of chicken broth.
Roast for another 20 minutes and baste again. Continue roasting as needed until the chicken reaches 165°F in the thickest part of the breast and thigh, without touching bone.
Rest the chicken for 5–10 minutes. Spoon the au poivre sauce over the top and serve.