Use the whisk attachment on a standing mixer on medium to whip the beef and pork for 1-2 minutes on medium until slightly sticky.
In a large bowl, whisk the eggs and milk together until combined and the add the bread crumbs; allow to soak for 5-10 minutes.
In a separate large nonstick or cast iron skillet, melt the butter on medium low and sweat the onions until they are translucent.
Add the salt, pepper, bread crumb mixture, and softened onions to the ground beef bowl and mix to thoroughly combine.
Roll 1/8 cup (or 2 Tbsp) into balls and either bake on sheet pans at 350°F for 30 minutes OR saute in butter (not in the recipe above, you'll need about 2 additional Tbsp) about 10 at a time in the large skillet on medium high for about 8 minutes a batch until browned all over.
Take out the meatballs and collect all of the juices from cooking in the large skillet (if using the pans, just pour into the skillet). On medium heat, saute the flour in the juices for about 2 minutes then add the cream and saute for about 2 minutes to make the sauce.
Pour the sauce on top of the meatballs, garnish with the tarragon, and serve immediately.