Mix mustard, honey, vinegar, Sriracha, paprika, pepper, and salt in a very large bowl until combined.

Place chicken, breast-side down, on a work surface. Starting at the tailbone, cut along either side of the backbone with scissors or a knife, cutting as close to the bone as possible. Discard the backbone and trim any excess fat or skin from the chicken.

Find the cartilage where the wishbone comes together and forms a "v" and make 1 shallow cut with a knife through the cartilage. Press down on either side to flatten the chicken and expose the breastbone, making another cut in the cartilage if necessary. Run your fingers along the breastbone until the rest of the cartilage pulls away. Make a few shallow cuts into the legs, thighs, and wings to ensure even cooking.

Transfer chicken into the bowl of marinade and toss until well coated. Marinate, breast-side down, in the refrigerator, 4 to 12 hours.

Preheat the oven to 425 degrees F (220 degrees C). Line a rimmed baking sheet with aluminum foil.

Transfer chicken, skin-side up, to the prepared baking sheet. Season with salt. Reserve marinade for basting.

Cook in the center of the preheated oven until chicken starts to darken in spots, 35 to 40 minutes.

While the chicken is roasting, transfer excess marinade into a small saucepan. Add chicken stock to thin the sauce a bit and bring to a boil. Remove from the heat.

Baste chicken with sauce and continue to roast, basting again if desired, until no longer pink at the bone and the juices run clear, about 15 minutes more. An instant-read thermometer inserted into the thickest part of the thigh, near the bone, should read 165 degrees F (74 degrees C).

Remove chicken from the oven and brush any rendered fat and accumulated juices from the pan onto the chicken.

Transfer chicken to a serving platter and garnish with green onions. Slice and serve, spooning any remaining sauce over top.