Melt the butter in a saucepan over medium–low heat and slowly whisk in mustard, soy sauce, vinegar, brown sugar, garlic, and the spices. Sauces can be frustrating when there's a high butter-to-oil ratio. But if the sauce breaks, you don't necessarily need to start over. Pour the sauce into a mason jar with a lid and shake the life out of it, as you would a cocktail.
Place the sauce in the refrigerator while you prepare the liver and onions. This allows the sauce to thicken to a better consistency.
Wash off the excess blood from the liver, and then slice it into half-inch medallions. Cut a yellow onion in half and slice it into rings or half rings. You can use however much onion you want, depending on the size of the liver. Just make sure you have enough for each bite of liver.
Make an egg wash to coat the liver and onions before applying the flour. Whisk together the buttermilk and two beaten eggs, then add a few dashes of hot sauce. Then, for the batter, you'll use the same flour mixture for the liver and the onions. Combine the flour, salt, black pepper, smoked paprika, and cayenne pepper.
Dip the liver and onions into the egg wash, toss them in the flour mixture until evenly coated, and then fry them in separate skillets on medium heat. I like to nearly submerge the onions in oil, but the liver should only be halfway under.
Fry the onions until golden brown, which should take about 5 minutes. Fry the liver for about 4 minutes per side.
You're done. Arrange the medallions as attractively as you see fit, throw the onions on top, and generously pour the sauce over everything. If you're trying to impress somebody, garnish with parsley.