Preheat the oven to 350°.
Boil the macaroni in a large pot of water with three tablespoons of kosher salt and drain it well.
Meanwhile, in a Dutch oven, on medium heat, melt 4 tablespoons of the butter and cook the onions until they're tender.
Add the flour to the onions and cook for two minutes. Then add the chicken stock and cream and cook for two more minutes. Add the salt and pepper. Remove the pot from the heat.
Stir a half cup of the sauce into a bowl with the beaten eggs. This is to temper, or heat up the eggs so don't scramble in the hot sauce. Mix it with a spoon, and then add the warmed up eggs to the warm pot. Stir in the macaroni and cheese, and put it in a greased 9 by 11 casserole.
In a small skillet, melt the rest of the butter. Make breadcrumbs out of the bread in the food processor, and cook them in the butter for 3 minutes.
Scatter the bread crumbs over the macaroni and cheese and bake for 30 minutes.