Whisk the cream, stock, yolk, salt, onion powder, and gelatin together. Let the mixture rest for 5 minutes to allow the gelatin to rehydrate. Add the panko, cheese, parsley, tarragon and lemon zest and mix to form a paste.
Use a fork to mix in the beef and pork using a cutting motion rather than a mashing motion. Be careful not to over mix. The mixture should be somewhat crumbly looking, and not smooth like a paste. Put the mixture in the fridge to rest.
To make the caramelized tomato sauce, heat a saucepan over medium heat and add a splash of oil. Pour in the the pureed onion and cook stirring occasionally until the mixture turns into a light brown paste. Add the wine and turn up the heat to high, letting the mixture boil down until there is almost no liquid left and the mixture is medium brown in color.
Turn down the heat to medium and add the tomato puree, sugar and salt. Cook, stirring occasionally until the mixture is reduced by about half and the sauce is thick and glossy.
Roll out the meat mixture into 3/4" balls then heat a non-stick pan over medium heat and add a splash of oil. Add the meatballs to the pan, being careful not to overcrowd the pan. Fry the until brown on one side, flip, then brown the other side. Slice a polpette in half to test for doneness.
Dip the cooked meatballs in the tomato glaze and serve.