1) Season and butter the turkey
Pat the turkey breast dry with paper towels. In a small bowl, mix the softened butter, garlic, lemon zest (if using), salt, pepper, paprika, thyme, rosemary, and onion powder.
Rub the butter mixture all over the turkey. If the turkey has skin, gently loosen it and rub some butter under the skin too. That is where the flavor magic lives.
2) Build the slow cooker base
Add the onion, celery, and carrot to the bottom of the slow cooker. Pour in the broth. This creates a flavorful rack so the turkey sits mostly above the liquid line instead of lounging in it.
Quick check: make sure the lid closes fully and the turkey is not wedged up the sides. Slow cookers are not into being overcrowded.
3) Slow cook
Place the turkey breast on top of the vegetables, skin side up. Cover and cook until the thickest part reaches 165°F. For a 3 to 4 pound bone-in breast, this is usually about 3 to 5 hours on LOW or 1 1/2 to 2 1/2 hours on HIGH, but always go by temperature and your specific slow cooker.
Thermometer tip: insert into the thickest center and avoid touching bone.
Avoid lifting the lid a bunch. The slow cooker hates that.
4) Rest (do not skip)
Transfer turkey to a cutting board and tent loosely with foil. Rest 10 to 15 minutes, then slice.
5) Optional: crisp the skin
If you want golden, crisp edges, preheat the broiler. Place the cooked turkey breast on a foil-lined sheet pan. Pat the skin dry with paper towels, then broil 3 to 6 minutes until the skin is browned and crisp. Watch it like a hawk, broilers go from “not yet” to “oops” fast.
6) Make quick gravy
Strain the liquid from the slow cooker into a measuring cup. Let it sit 5 minutes, then skim off excess fat from the top.
In a small saucepan over medium heat, melt 2 tablespoons butter. Whisk in flour and cook for 1 minute. Slowly whisk in 1 cup of strained cooking liquid. Simmer until thickened, 2 to 4 minutes. Add Worcestershire or soy sauce if using.
If it is too thick, whisk in more cooking liquid (up to 1 1/2 cups total) or a splash of broth. If it is too thin, simmer a little longer. Taste and season with salt and pepper.
7) Slice and serve
Slice turkey against the grain. Spoon gravy over the top and serve while everything is still cozy.
