Cover shank, onion and heart with the water, cover with a lid, then simmer or bake at 300 for 2 hours or until very tender. Chill the shank and spleen until cool enough to handle. Reserve the cooking liquid, there should be exactly 4 cups, if there isn’t, add enough water or stock to make up the difference.
Remove the meat from the shank bone, trim the heart with a sharp paring knife, then put the heart, liver, onion and shank in a meat grinder, or puree in a food processor until fine. Sweat the meat mixture to evaporate the water in a saute pan for 2-3 minutes, then reserve, and allow to cool, covered.
Meanwhile, Whisk the cornmeal and buckwheat flour with the salt and spices, and combine with the 4 cups of cooking liquid in a sauce-pot big enough to hold it. Heat the mixture, whisking constantly until it thickens, then turn the heat down to low and cook for 15 minutes.
Add the cooked meat to the cornmeal mixture, and continue cooking, stirring occasionally to prevent sticking on the bottom for another 30 minutes. You can also bake the mixture at 300 for 30 minutes, or until very thick, as long as it’s covered.
When the mixture is very solid and the cornmeal is well cooked, pour the mixture into a greased loaf pan, there may be a small amount of excess. Chill the scrapple, uncovered on a counter until room temperature, then refrigerate until completely set before slicing.
To serve the scrapple, cut ½ inch slices and fry until crisp on both sides, and serve for breakfast or lunch, with maple syrup on the side, if desired.