Pat the turkey breast completely dry with paper towels and season generously with sea salt and black pepper on all sides. This crucial step ensures proper browning and helps the herbs adhere better to the meat's surface.

Heat 2 tbsp butter in a large skillet over medium-high heat until foaming. Carefully place the turkey breast skin-side down and sear for 4–5 minutes until golden brown, then flip and sear the other side for 3–4 minutes—this creates a beautiful, flavorful crust.

Transfer the seared turkey breast to your slow cooker and arrange the lemon slices, onion quarters, and minced garlic around it. Scatter fresh rosemary and thyme on top, then add bay leaves.

Pour the white wine and chicken broth into the slow cooker, ensuring the liquid reaches halfway up the turkey breast's side. Dot the top with the remaining 2 tbsp butter, cover, and cook on LOW for 6 hours—an elegant slow cooker turkey breast recipe requires precise timing to achieve optimal tenderness without overcooking.

Remove the turkey breast to a cutting board and tent loosely with foil to rest for 10 minutes. Strain the cooking liquid into a saucepan, discarding solids.

Sauté the sliced mushrooms in the strained liquid over medium heat for 3–4 minutes until tender. Mix cornstarch with water to create a slurry, then whisk into the pan for 1–2 minutes until the sauce thickens beautifully and coats the back of a spoon.

Slice the turkey breast against the grain into elegant portions and arrange on a serving platter. Drizzle generously with the mushroom sauce and garnish with fresh herb sprigs.