Remove the turkey breast from the fridge 30 minutes before cooking to bring it to room temperature. Cut 3 pieces of kitchen twine and set aside.
Add the butter, parsley, lemon zest, thyme, onion powder, garlic, 1/4 teaspoon salt and 1/4 teaspoon pepper to a small bowl. Use a fork or a rubber spatula to incorporate all of the ingredients into the butter. Set aside.
Pat the turkey breast dry. Using clean hands, starting at one end of the breast, slide your fingers under the skin to create a pocket between the meat and the skin. Use your hands to transfer the butter into the pocket and smooth the butter into an even layer.
Use the kitchen twine to tie the turkey horizontally in 3 places. This will help the turkey breast cook evenly and remain juicy. Flip the turkey breast so that it is skin-side down. Drizzle it with 1 teaspoon olive oil and sprinkle with 1/2 teaspoon salt and 1/2 teaspoon pepper. Flip the turkey over and repeat the same seasoning process.
Arrange the onion slices on a small rimmed baking sheet. Pour 1/3 cup water into the baking sheet and place the turkey breast on top of the onions. Transfer to the oven and roast for 1 hour. Remove the turkey from the oven and baste it with the butter and juices from the baking sheet. Continue to roast until the internal temperature is 160 degrees F, another 20 to 25 minutes. Allow the turkey to cool for 15 minutes. (In this time the internal temperature will rise another 5 degrees.)
Slice the turkey and serve it with the roasted onions.