Combine the fennel, black pepper, kosher salt, crushed red pepper flakes, garlic, rosemary, sage, lemon zest & juice, orange zest and juice and olive oil. Whisk to combine. Set aside.
Use a pair of kitchen sheers to trim away the back portion of the rib cage and back bone, leaving just the turkey breast intact. To do this, flip the turkey so the back is facing up. Let the natural line of where the breast meets the rib cage be your guide. Start on one side and cut along the bottom of the breast (so you're not removing any of the breast meat) and cut through the ribs from the butt end to the neck portion, until you get to the point where the bones are too tough for the sheers.
Use the same method on the opposite side of the turkey breast. Grasp the the portion that you're cutting away with one hand and the breast meat side with the other hand. Leverage the back bone back until you hear a satisfying "crack". Then use the poultry sheers to cut between the joints and remove the back portion. Now you have a turkey breast without the back bone. Don't toss the back bone, you can use it for your gravy stock.
Place the turkey breast in a one gallon zip top plastic bag and pour the marinade over it. Tip the bag to and fro to coat in the marinade. Refrigerate the turkey for at least 4 and up to 12 hours. Don't marinate longer as the acidity from the marinade can break down the flesh and make it mealy.
TO COOK THE TURKEY:
Remove the turkey from the refrigerator about 45 minutes before roasting, so it can come closer to room temperature. If you put a cold turkey in the oven, it will just take longer to cook.
Preheat the oven to 325°. Scatter the onion slices in a single layer in the bottom of a roasting pan.
Remove the turkey from the marinade. Wipe away the excess zest, garlic etc. and pat it dry with paper towels. Rest the turkey breast on top of the onions. Add the chicken broth to the bottom of the pan and cover the pan tightly with aluminum foil. Place the turkey in the oven for 45 minutes to an hour (Depending how big your turkey breast is).
Remove the foil and turn the heat up to 425°. Brush the turkey breast with olive oil and continue to cook until the thickest part of the turkey breast registers about 165°. Use an instant read thermometer to be sure of the turkey temperature. Remove from the oven and transfer to a cutting board. Tent lightly with foil and let it rest for 15 minutes for the juices to redistribute and settle before carving.
Drain the pan juices and drippings into a glass measuring cup for the gravy. (If you want the onions in your gravy, leave them in the pan. If you don't want the onions, discard them.
FOR THE TURKEY STOCK:
While the turkey is roasting, in a large saucepan add the turkey back, onion, celery, carrot, black peppercorns, kosher salt and bay leaf. Cover with 4-5 cups of water and bring to a boil over high heat. Place the lid on the pan so it's slightly askew and steam can escape. Reduce the heat to a low simmer and cook for an additional 20 minutes.
Remove the turkey back from the pot and set aside to cool. Strain the turkey stock by placing a strainer over a bowl and pouring the contents through the sieve. Discard the solids and use the turkey stock later to make the gravy.
You can use bits of meat from the turkey back in the gravy if you want. To do this, wait until the turkey back has cooled, remove any skin and bones. Pick off any meat and transfer to a small bowl until you're ready to make the gravy.
FOR THE GRAVY:
Add butter to the roasting pan and melt over medium high heat. Sprinkle the flour salt and pepper over the butter and stir to form a paste (called a roux). Cook the roux for about one minute until bubbly and fragrant (It's important to cook the butter/flour mixture to cook off any raw flour taste).
Add the reserved pan drippings in quarter cup additions to the roux, whisking or stirring constantly after each addition until well blended. Add the stock in the same manner until there's just one cup of stock left. You can add the last bit of stock all at once and whisk constantly while bringing the gravy to a boil over medium high heat. Boil for one minute until it thickens.
Taste the gravy for seasonings and adjust as necessary. Note: if you'd like a darker gravy, you can add ½ to 1 teaspoon of Gravy Master.
SERVE THE MARINATED TURKEY:
When the turkey has rested, slice the breasts off the bone and cut them crosswise into 1/4" thick slices. Arrange on a platter. Serve with gravy.