I'm thinking of roasting a turkey breast part-way in the oven, then resting it (skin side up) on a bed of homemade stuffing in a casserole dish to finish the roasting.
Any reason this wouldn't work?
It would work, but you just as well start it on the bed of stuffing. Jacques and Juila do a partially disassembled turkey that way and Cook's Illustrated does this with spatchcocked chicken. CI puts a foil "boat" under it all to hold things together more, but it's not required.
http://www.washingtonpost.com/wp-dyn/content/article/2009/11/17/AR2009111700746.html will explain Jacque't technique.
If cooked separately, you're not wrong, But with the bird above the stuffing, you slow down how much heat it's getting and all the juices run through the stuffing keeping it from drying out or overcooking.
Many years back, Dad roasted a "deconstructed" T-Day meal... WAY before anyone coined that phrase. Cooking for a BIG crowd at our house. HE preferred white meat, so bought 2 whole breasts... and then parts... some thighs/legs & wings. We made eavy-duty packets to hold the stuffing that would normally have been IN the bird. Left open on top with breast set on top. Could still baste without stuffing getting drowned. Ended up with lots of good brown "gunk" in roasting pan for gravy. Stuffing wasn't quite like IN the bird, but better than in a casserole... imo!?!
I'm going to try it with a butterflied whole roasting chicken. At the supermarket, I couldn't believe the difference in cost. For about 7.5 lbs, the chicken was about $7, and the turkey breast over $27. AND the chicken was "minimally processed" while the turkey had that cheap broth/salt water they like to inject.
Or maybe I'll do two casserole dishes, one with parts that take longer. Then I can time them separately.
While you are butterflying the fowl, take a bit of additional time and debone it. Serving will be much easier.