Place approximately 18 inches of parchment on your countertop. Place chicken on top and use parchment as your prep surface. (Alternately, you can prep chicken in your sink.) Remove giblets from chicken, if any. Rinse chicken with room temperature water, making sure to rinse out the cavity. Pat thoroughly dry with paper towels or a dish towel.
In a small bowl, combine salt, paprika, pepper, oregano, onion powder, and garlic powder.
In another small bowl, mash together 1 tablespoon spice mixture and 1 tablespoon softened butter into a paste (you can microwave 10 seconds to help combine). With your fingers or a small knife, starting at the cavity, carefully loosen skin of chicken breasts, separating it from the meat. Rub paste evenly under breast skin. Rub remaining tablespoon butter, then remaining spice mixture, over the entire outside of the chicken.
Stuff chicken cavity with half of the onion chunks, garlic head halves, and 2 lemon quarters. Set aside anything that doesn’t fit into the cavity.
Instant Pot (IP) version:
Turn on an exhaust fan and place IP nearby. Add avocado oil to IP and press “saute” on “high”/”more.” Once oil is hot, add chicken, breast-side down, and sear 5 to 7 minutes. Using a pair of tongs and a fork, flip chicken. Sear another 5 minutes.
Once chicken is seared on both sides, press “cancel” and add broth, remaining onion chunks, and anything that didn’t fit in the cavity. (Try not to pour broth directly on top, which can wash off spices). Cover and lock, turn to seal, then set to manual for 24 minutes. Each pound of chicken takes about 6 minutes in the IP, so adjust accordingly if your chicken is larger or smaller than 4 pounds.
Allow IP to come to pressure naturally, approximately 15 minutes. Remove chicken carefully using tongs and a fork and cool 10 to 15 minutes; if preferred, squeeze remaining lemon quarters over chicken before serving.
Turn on exhaust fan. Place a large sauté pan over high heat and add avocado oil. When oil is hot add chicken, breast-side down, and sear 3 to 4 minutes. Using a pair of tongs and a fork, flip chicken. Sear another 2 to 3 minutes, then carefully transfer chicken, breast-side up, to slow cooker. (Try not to transfer the leftover oil with it, since it will likely have burnt spices in it.)
Add broth, remaining onion chunks, and anything that didn’t fit in the cavity to the slow cooker (try not to pour broth directly on top, which can wash off spices). Cover and turn crockpot to “low.” Cook for 6 to 7 ½ hours, carefully flipping chicken halfway. If preferred, squeeze remaining lemon quarters over chicken before serving.