To slow cook a turkey in the oven overnight, you use a low temperature (about 180–200°F / 80–95°C), cover the bird so it doesn’t dry out, and rely on a meat thermometer to confirm it reaches a safe internal temperature of at least 165°F (74°C) in the breast and 175–180°F (79–82°C) in the thigh.

Safety and timing

• Use only a fully thawed turkey; never slow roast a frozen or partially frozen bird because it will sit too long in the “danger zone” (40–140°F / 4–60°C).
• Plan on roughly 9–10 hours at 180–200°F (for about a 12–16 lb / 5–7 kg turkey), but treat time as a guideline and temperature as the rule: the thickest part of the breast should reach at least 165°F and the thigh 175–180°F.

• Use a reliable oven and an oven-safe meat thermometer; if your oven’s minimum is 200°F, use that and begin checking earlier.

Prep steps in the evening

1. Thaw and dry: Make sure the turkey is fully thawed, remove giblets, and pat very dry with paper towels to help browning later.

2. Season: Rub the outside and under the skin (especially the breast) with softened butter or oil mixed with salt, pepper, and herbs like rosemary and sage; lightly season the cavity as well.

3. Pan setup: Place the turkey breast side up on a rack in a deep roasting pan; add 3–4 cups of water or broth to the bottom to create a moist environment and drippings for gravy.

4. Cover: Tent the entire pan tightly with heavy-duty foil, sealing the edges well to trap steam and prevent drying.

Overnight roasting method

1. Start hot, then go low (optional but helpful):
   • Place the covered turkey into a 375–400°F (190–200°C) oven for about 45–60 minutes to bring the bird quickly through the most unsafe temperature range and start browning slightly under the foil.

 * Then reduce the oven to 180–200°F (80–95°C) without opening the foil.

2. Slow cook overnight:
   • Roast 8–10 hours at the low temperature; for example, put the turkey in around 10–11 p.m. and plan to check it between 7–8 a.m. for a 12–16 lb bird.

 * Avoid opening the foil during the night; every time you open the oven, you lose heat and extend the cooking time.

3. Check doneness in the morning:
   • Insert an instant-read thermometer into the thickest part of the thigh and breast without touching bone; once the breast is at least 165°F and thigh 175–180°F, the turkey is safely cooked.

 * If it is not at temperature, re-cover and continue at 200°F, checking every 30–45 minutes.

Browning and holding until serving

• Crisp the skin: Once the turkey is at temperature, increase the oven to 425–450°F (220–230°C), remove the foil, and roast 15–25 minutes until the skin is deep golden and crisp.

• Rest: Remove from the oven, tent loosely with foil, and rest at least 20–30 minutes before carving so the juices redistribute.

• If serving later: After resting, you can carve the turkey, refrigerate the slices covered, then reheat in a 300–350°F (150–175°C) oven covered with foil and a little broth until just hot, to avoid drying out.

Quick tips and variations

• Breast-side-down option: Some cooks roast breast side down during the long low cook so juices run into the breast, then flip breast side up for the final high-heat browning.

• Oven bag option: A roasting bag at 250°F for about 7–8 hours (for a medium bird) can also yield very moist meat; still verify internal temperatures as above.

• Gravy: Use the pan juices (after skimming fat) as the base for gravy by thickening with a simple roux of fat and flour on the stovetop.