To make the bacon butter: Pulse the butter, chopped bacon, and herbs until smooth in a food processor fitted with the blade attachment.
Remove the giblets and neck from the turkey's cavity. Save the neck for making stock and the giblets for making gravy. Set the turkey breast-side up on a roasting rack set inside a roasting pan. Liberally rub the turkey inside and out with the bacon butter. Put the onion pieces in the cavity of the turkey. Tie the turkey's legs together with a piece of kitchen twine, if desired.
Wrap each leg with 2 slices of bacon. Wrap each wing with 1 slice of bacon, tucking the ends underneath the bird or in the crease between the leg and the breast.
Lay 8 slices of bacon side by side on a piece of parchment or waxed paper. Make sure the slices are as close together as possible. Working from the bottom up, weave another 8 slices of bacon into the first layer. Again, make sure these slices are as close together as possible.
Place the woven bacon onto the turkey breasts: Set the piece of paper next to the roasting pan and line the bacon blanket up at an angle to create a diamond. Carefully flip the paper over the turkey breast. Don't worry about aligning it perfectly — you will still be able to move the blanket a little once you get it into place. Remove the parchment and tuck the blanket in and around the breast and into the leg crease. Trim any excessive overhanging bacon with scissors, if desired.
Heat the oven to 425°F Pour 2 cups of water into the roasting pan underneath the turkey. This will create steam in the oven and prevent the bacon from rapidly shrinking and falling off the bird. Roast the turkey for 30 minutes to darken and crisp the skin.
Reduce the oven temperature to 325°F and roast for an additional 2 to 2 1/2 hours. The rule of thumb for cooking a turkey is 13 minutes per pound, but more important than time is the temperature of the turkey.
Use a probe thermometer to test the turkey in 3 places: the breast, the outer thigh, and between the thigh and breast. The turkey should reach 165°F in all 3 places.
Carefully remove the turkey and the roasting rack to a cutting board or rimmed baking sheet. Rest the turkey for at least 30 minutes before carving. Use the drippings to make gravy, if desired.