Freeze the beef heart, kidney, and ground beef (or thaw, if they're already frozen) to about 32°F, but to no cooler than that.
Trim the harder fat from the frozen beef heart.
Cut the frozen heart into 1” slices.
Grind the beef heart, kidney, and ground beef together, before they thaw too much.
Put the ground meats into a freezer bag and distribute in the bag to an even thickness.
Refreeze the ground meats, again to about 32°F.
Grind the refrozen meats a second time.
Set the pot over low heat and melt the lard and butter in the saucepan. When the fats are melted, add the ground heart, kidney, chuck, the garlic and ground mustard and stir well.
Let the sauce simmer for 45 minutes, stirring occasionally. Don’t allow the meat to dry out; add a tablespoon each of butter and lard if necessary and lower the heat when necessary. At the end of the 45 minutes, add the chili powder, and add the ground cumin to taste. Also add salt and pepper to taste, then simmer the sauce another 5 minutes before serving.
For best results, serve on grilled Koegel Viennas that have been cooked over low heat (250F) so the natural casing snaps when bitten.
Notes
Don’t use garlic powder instead of minced garlic. Throw that powdered stuff away … it’s not the same. However, granulated garlic is an excellent substitute for minced garlic.