Melt 1 tablespoon butter in a skillet over a medium heat. Add the onions and cook until softened and golden, 15 to 20 minutes. Tip the onions out of the pan and onto a plate to cool.
Combine the breadcrumbs with the cream in a large bowl and let them sit until they soak up most off the cream and swell, 3 to 5 minutes. (The bread may not soak up all the cream and that's okay.)
Add the cooled onion and the egg to the breadcrumb mixture and mix everything together well. (If you prefer a less crumbly meatball, first squeeze the soaked breadcrumbs gently and drain the excess cream.) In a separate bowl, mix the beef, veal, and pork until thoroughly combined. Add the meats to the bread mixture, season well with salt and pepper, and then mix everything together, preferably with your hands.
Shape the meatballs mixture into balls the size of a small walnut using a heaping tablespoon for each. You should have about 20 meatballs.
Melt the remaining tablespoon butter in a large https://leitesculinaria.com/product/valencia-non-stick-skillet-set over a medium heat. Fry the meatballs, working in batches if necessary, and turning as necessary, until brown all over, about 15 minutes total. You may need to add more butter to the skillet. Serve plain, with a dipping sauce, atop spaghetti, or however else you please.