In a large pot or container, combine 8 cups water, kosher salt, brown sugar, peppercorns, and bay leaves. Heat just until salt and sugar dissolve (about 3–5 minutes), then remove from heat and let cool to room temperature.

Once the brine is cool, add crushed garlic and thyme sprigs. Add 2 cups ice to chill the brine further.

Place the turkey breast in a resealable brining bag or a container and pour the cooled brine over it, ensuring the breast is submerged. Refrigerate for 6–12 hours (overnight is fine for 3-lb breast).

After brining, remove the turkey breast, discard brine, and pat the breast thoroughly dry with paper towels. Let rest uncovered in the refrigerator for 1 hour if time allows to dry the skin for better browning.

Roast Seasoning & Basting

Preheat the oven to 425°F (220°C). Position a rack in the center.

In a small bowl, combine softened butter, olive oil, chopped rosemary, chopped thyme leaves, lemon zest, minced garlic, 1 tsp kosher salt, and 1 tsp freshly ground black pepper. Mix until smooth.

Gently loosen the skin over the turkey breast with your fingers to create a pocket (do not tear). Spread about half of the herb butter directly under the skin over the meat. Rub the remaining butter over the top of the skin to coat evenly.

Place the quartered onion, carrot chunks, and celery in the bottom of a roasting pan to create a bed and flavor the drippings. Pour 1 cup of stock into the pan.

Set the breast skin-side up on the vegetables and tuck any loose skin. Brush any residual butter from the bowl over the breast.

Roast at 425°F (220°C) for 15 minutes to develop color, then reduce oven temperature to 350°F (175°C) and continue roasting.

Roast until an instant-read thermometer inserted into the thickest part of the breast (avoid touching bone) reaches 160°F (71°C). For a 3-lb bone-in breast this typically takes 50–70 minutes total roasting time (including the initial high-heat period). Start checking at 40 minutes after reducing heat.

Every 20 minutes after reducing the heat, baste the breast with pan juices. If the pan becomes dry, add up to 1/2 cup more stock.

When the thermometer reads 160°F (71°C), remove the turkey breast from the oven and tent loosely with foil. Let rest for 15–20 minutes; the internal temperature will rise to about 165°F (74°C) and juices will redistribute.

Carving and Serving

Transfer the breast to a cutting board. Using a sharp carving knife, slice the breast across the grain into 1/4–1/2-inch thick slices.

Serve slices with pan juices spooned over top or strained into a simple gravy (optional).