Combine kosher salt, black pepper, onion + garlic powder in a small bowl.
Transfer 1/4 tsp of salt mixture into another small bowl, then add to it 3 tbsp of the chopped herbs and 1-1/2 tsp of olive oil.
Place the hen on a cutting board, breast side down. With a sharp knife or pair of kitchen shears (my preferred tool for this job), begin cutting through the bird on either side of the back bone. If you are cutting through the ribs, it should cut away easily.
Do this along both sides to remove the entire backbone. Reserve the back bone and cut into smaller pieces for making pan sauce later.
Lay the bird, skin side up and with the heel of your hand, press down the breast bone to flatten it. You should feel the bone/cartilage underneath give or hear a little pop.
Separate the skin from the breast meat. Stuff half of the herb mixture between the skin and breast meat. Repeat with the other other hen. Rub olive oil all over the skin of the birds and season with the reserved salt mixture.
Preheat oven to 350°F. On a baking tray, create an x with rosemary, and add some lemon slices. Set aside.
Place a large skillet over a medium burner. Add 1-1/2 tsp olive oil. Working with one a time, place a hen, skin side down in the skillet and allow to brown for 3 to 4 minutes. Take this opportunity to season the inside of the bird with the salt mixture.
Place each hen, skin side up, on top of one of the rosemary x's. Point the legs to the outside ends of the tray. Roast for 45 to 50 minutes or until the thickest part of the thigh registers 165°F.
While the hens roast, make the pan sauce. To the same skillet, add the cut up neck bones and sauté for about 5 minutes over medium heat. Remove from the pan and discard.
Add the shallots and cook until tender and translucent. Next, add the garlic and cook for another minute. Add the white wine and chicken broth all at once to deglaze the skillet. Allow it to simmer and reduce by half.
Into a bowl or small pitcher, pour the sauce through a fine mesh strainer to remove all the solids. Then add butter and stir to melt. Taste for seasoning and toss in the remaining tablespoon of fresh chopped herbs.
When the birds are done roasting, allow them to rest for at least 5 minutes. Split each hen in half straight down the breastbone. You can whisk any accumulated juices from the baking tray to your pan sauce.