Step 1Mix the paprika, garlic powder, celery salt, onion powder and kosher salt in a bowl. Rub this spice mixture onto the brisket on both sides. Really rub it in there like you are massaging it. Cover and let chill overnight.
Step 2Heat the oven to 300 degrees. Heat a braising pan over high heat and add 3 tablespoons oil. When it's smoking, add the brisket carefully and brown deeply, about 3 minutes each side.
Step 3Add one-half cup water and cover with a lid. Place in the oven, checking periodically to make sure the water has not boiled away, and roast 1 1/2 hours.
Step 4While the brisket roasts, heat a skillet and add the remaining 1 tablespoon oil and the onions and slowly caramelize, cooking on low heat, stirring occasionally, until golden brown, about 40 minutes. Set aside.
Step 5After 1 1/2 hours, add the onion mixture to the brisket. Cook another 1 1/2 hours until very tender.
Step 6Remove the brisket from the pan to cool. Cook the pan juices if necessary to reduce, until the onion mixture is thick and gooey. Set aside to cool to room temperature.
Step 7Pour off any grease from the onion mixture. Chop one-fourth of the brisket finely by hand (food processor will turn it into paste) and place it in a bowl, reserving the remaining brisket for another use.
Step 8Add the onion mixture, egg yolk, nutmeg and parsley. Season with salt and pepper.