Place the beef feet and neck bones on a baking sheet or in a roasting pan, and place the sheet or pan on the middle rack of an oven preheated to 425°F. Roast feet and bones for 45 minutes or until nicely browned.
Transfer the feet and bones to the liner of a slow cooker.
Deglaze the baking sheet or roasting pan, and add the deglazed liquid to the slow cooker.
Scrape the baking sheet or roasting pan free of any bits of fond. (Fond is the brown bits still left on the pan after deglazing.) Transfer this fond to the slow cooker.
Add just enough water to the slow cooker to cover the bones.
Add Marsala to the slow cooker.
Allow the feet and the bones to soak in the acidulated water for one hour.
After one hour, turn the slow cooker to high and bring the mixture to a boil. Skim off any scum that rises to the top.
Add the vegetable scraps or vegetables along with the bay leaf and peppercorns.
Add additional water to cover everything, but DO NOT add any more water than needed.
Cover the slow cooker and turn the temperature to Warm or Low. You will have to experiment with the temperature since every slow cooker is different.
You only want to see small bubbles in the bone broth as it simmers. You DO NOT want your bone broth bubbling.
Simmer the bone broth for 12 hours.
After 12 hours, remove the feet, bones, and vegetables from the slow cooker and strain the bone broth. (See video.)
Decant the bone broth and refrigerate or freeze. Your bone broth will stay fresh for one week refrigerated without the fat cap or two weeks with the fat cap in place. (See video.) You can freeze the bone broth for up to 12 months.