In a large stockpot over medium heat, sauté onions, celery, and garlic in olive oil. When they have softened, after 5 minutes, add the bones and fill the pot with 5 quarts cold water. Add the bay leaves, peppercorns, and apple cider vinegar, and bring to a boil. Skim any brown foam that rises to the top with a fine mesh strainer or slotted spoon. Turn heat to low/medium-low so that broth is simmering gently. Set a slightly askew lid on top of the stockpot. Cook, stirring and skimming every hour or so, 6 - 8 hours. Add sea salt when there's about an hour of cook time left – just enough to bring out the flavor.
When you are satisfied that your broth has achieved its maximum potential, strain through a mesh strainer. Pour into clean quart jars, allowing a full 2-inches of head space if you plan to freeze the broth. Set jars in the fridge right away and cool completely in the fridge overnight, then label and freeze. Frozen broth will keep for 6 months, especially if you push it to the very back of your freezer.