Preheat the oven to 400°F (200°C).
Place the beef marrow bones on a baking sheet and roast in the oven for 30 minutes, turning halfway through until browned.
Transfer the roasted bones to the slow cooker.
Add the quartered onion, chopped carrots, chopped celery, smashed garlic, bay leaves, black peppercorns, and apple cider vinegar to the slow cooker.
Pour 10 cups of cold water into the slow cooker, covering the bones and vegetables.
Cover and cook on low for 12 hours.
After 12 hours, remove the bones and vegetables from the broth using a slotted spoon.
Strain the broth through a fine mesh sieve or cheesecloth into a large bowl or pot.
Stir in 1 teaspoon of salt and adjust seasoning if needed.
Let the broth cool to room temperature, then refrigerate or use immediately.