Brine the turkey
Mix water, salt, and sugar until dissolved. Add the turkey breast. Cover and refrigerate for 4–6 hours.
Dry the turkey
Remove from brine. Rinse lightly and pat very dry with paper towels.
Make the herb butter
In a bowl, mix butter, lemon zest, lemon juice, thyme, garlic, and black pepper.
Season the turkey
Gently loosen the skin. Rub most of the butter under the skin. Spread the rest on top.
Prepare the pan
Place onion slices in a roasting pan. Set the turkey on top. Add lemon slices and thyme around it.
Roast
Preheat oven to 375°F (190°C). Roast uncovered for 60–75 minutes until internal temp reaches 165°F.
Rest and slice
Let rest for 15 minutes. Slice and serve.