Preheat the oven to 375 degrees. Cook the bacon in a large ovenproof skillet over medium heat until crisp, about 7 minutes; transfer to a paper-towel-lined plate. Add the onions to the pan with 1 teaspoon salt and some pepper; cook over low heat until tender, about 5 minutes, scraping up the browned bits with a wooden spoon. Set the skillet aside.
Combine the cornbread, spinach, cheese, pecans, garlic, 1 tablespoon of the mustard, 1/2 teaspoon salt and the nutmeg in a medium bowl. Crumble in the bacon and toss gently. Stuff the mixture between the skin and flesh of each chicken half, making sure to cover the breast and thigh.
Place the chicken halves skin-side up on top of the onions in the skillet. Combine the butter and honey in a bowl and brush all over the chicken; season with salt. Roast in the oven until a thermometer inserted in the thigh registers 165, about 50 minutes. Transfer the chicken to a cutting board; let rest for 10 minutes. Whisk the remaining 1 tablespoon mustard into the onion mixture and set aside. Carve each chicken half and serve on a platter with the onions.