Add all the ingredients for the meatballs to a large bowl: ground meat, 1/3 of the caramelized onions (roughly chopped), garlic pressed through a garlic press, thyme, milk, eggs, breadcrumbs, grated Gruyere, salt and pepper. Mix the ingredients until well-combined using your hands or a stand mixer fitted with a paddle attachment (just don´t beat the mixture for too long - until just combined).
If you´re not sure about the seasoning, you can cook a small amount of the meat mixture in the pan until cook through, taste it and add more seasonings if necessary.
Using an ice cream scoop, scoop the mixture into rough balls on a prepared baking sheet or wooden board. Coat your hands in olive oil (to prevent sticking) and roll the mixture into meatballs.
Heat the frying oil in a large frying pan (you can use the same pan you´ve used to cook caramelized onions). Cook the meatballs over medium/medium-high heat until browned on all sides, they don´t have to be cooked through, they can finish cooking later in the sauce. Transfer the meatballs to a plate.