Start the Traeger smoking according to the manufacturer's instructions.
Once smoking, set the Traeger's temperature to 225°F and let come to temperature while you prep the turkey.
Make the Herb Butter
Soften the butter until it is easy to mix but not melty.
Mix in the herbs de provence a few pinches at a time, adding until the butter seems fairly well saturated.
Add freshly ground pepper until it also evenly saturates the butter.
Add a few generous pinches of salt. (I always use course Kosher salt, but any will do.)
Finish off by adding the lemon juice, adding slowly so as to not make the mixture too runny. (You want the butter to be thick enough to spread it under the skin, so it should still be a soft solid after you mix everything together.)
Prep the Turkey
Pull the turkey breast out of the fridge 30 minutes to 1 hour before you plan to start smoking.
When you are ready to start, remove from packaging, pat dry, and trim any excess skin or fat from around the edges.
Use your hand to gently separate the skin from the meat.
Spread the herb butter under the skin, so that it sits directly on the meat. Transfer it with your hands, and then you can press on the outside of the skin to move it around and get full coverage.
Salt and pepper the outside of the turkey breast as well.
Smoke the Turkey
Place the prepped turkey breast directly on the grill of the preheated Traeger.
If you want to, you can insert a thermometer now in order to track the temperature as the breast cooks. You can also just check it a few hours later and then monitor as needed.
The turkey is done when a meat thermometer inserted near (but not touching) the bone registers 165F°. In theory, a turkey breast should take ~ 30 minutes / pound, so 2.5 hours for this 5 pound breast. However, I ended up smoking at 225°F for 2 hours, and then cranking the heat up to 425°F for another hour to get from an internal temp of 130°F to 165°F in time for dinner.
Let the breast rest for at least 20 minutes, and up to 1 hour, to maintain maximum moisture.