This preparation will NOT produce a turkey breast with crispy skin. In fact, I normally remove the skin completely (and discard it) after the turkey has cooked. The meat comes out wonderfully moist and the gravy is out-of-this-world!
Adjust your oven rack to the lowest position. Preheat oven to 250°F.
Rinse and dry the turkey breast. Even when I purchase a brined turkey breast, I still like to lightly salt the turkey breast under the skin. You, of course, do not have to. If the turkey breast that you purchase has NOT been brined or koshered, liberally salt the breast under the skin.
Heat a large pot over medium-high heat with the oil. When hot, add the turkey, breast-side down. Add the onion around the edges of the bird. Add the garlic on top of the onions. Cook several minutes or until the skin has browned. Flip the bird over so that the breast is facing upward. Stir the onions and garlic. Cook a couple minutes longer. If you have a probe thermometer, insert it into the thickest part of the breast – making sure the probe is not sticking up in the air in a way that would impede your covering the turkey breast with a lid. Cover the pot with aluminum foil and top with a tight-fitting lid. Crimp the foil around the seam of the pot to ensure steam does not escape. Cook until the internal temperature reaches 160F – roughly 2 1/2 hours.
When the turkey is done, remove it from the oven and flip the bird over so that the breast is submerged in the accumulated juices. Cover and let rest 15 minutes. During this time, the internal temperature will increase to at least 170°F.
Remove the turkey from the pot and place it on a large platter. Keep it tented so it stays warm. Strain the liquid and solids from the pot into a large bowl. Use the back of a ladle to make sure you smash the onion/garlic to extract all of the juices. Discard the vegetable solids. Skim some or most of the fat that settles on top of the broth and discard any unwanted fat. Measure how much turkey broth you have. If you do not have at least 4 cups, add some chicken stock to come up to a total of 4 cups of liquid. If you like particularly thin gravy, you can add an extra 1/4 to 1/2 cups of turkey/chicken broth. If you like thicker gravy, reduce the liquid by 1/4 to 1/2 cup.
To make the gravy, heat a saucepan over medium heat with three tablespoons of butter. After the butter has melted, add the flour and whisk to combine. Cook the butter/flour mixture for about 5 minutes or until it is brown in color. Add the turkey broth and whisk to make sure there are no lumps. Bring to a boil then simmer about 5 minutes. Too thin? Simmer another few minutes or until the gravy is your desired thickness. Too thick? Add more turkey broth, chicken broth, or even water. When the gravy is your desired level of thickness, taste it and add some salt and pepper, if needed. You'll probably need at least some salt.
Serve the turkey breast and gravy with some mashed potatoes.