Buttermilk Baked Chicken

8 chicken thighs, with bones and skin on (I've also made this with legs to good effect)
1 quart buttermilk
1 small onion, coarsely sliced
2 Tbsp sea salt
2/3 cup pamelas baking mix
1 tsp freshly ground pepper
1 or 2 fat pinches cayenne pepper
2 Tbsp butter
splash grapeseed or olive oil
salt and pepper for serving

Rinse and pat dry chicken, place in a large bowl. Toss onions on top, and pour in buttermilk. Add cold water until chicken is covered. Dissolve 1 Tbsp of the salt in a little warm water, pour over chicken mixture. Give a stir to combine and refrigerate overnight (or at least 6-8 hours, so put it in before you go to work, or in the morning if you don't work). Preheat oven to 400 degrees. Add butter and splash of oil to a baking dish large enough to fit the chicken in one layer, and place baking dish in oven while it preheats, until butter is melted completely. You want your pan to be really good and hot, don't skimp on the preheating time.Pour 1 Tbsp of salt, the two kinds of pepper, and the flour in a large ziploc bag, seal and shake to combine. Remove chicken from buttermilk mixture and pat completely dry. Place two pieces of chicken at a time in the ziploc bag, seal shut, and shake to thoroughly coat. Shake off the pieces of chicken to remove excess flour--this is important or you could end up with a mushy coating. Line up on a cutting board or a plate after coating.Place chicken in hot pan, skin side down, and bake until the skin is a deep golden brown (this takes at least an hour to an hour and fifteen minutes, on average). Flip chicken and cook until the same color is achieved on the other side (about 20-30 minutes). Remove to serving platter, sprinkle with sea salt and freshly ground pepper before serving.