First, let's repeat the rule of cooking (*) lentils: we need raw, dried lentils, not canned ones, and we will only partially cook them. Any kind of lentil will be fine, as long as it is raw.
Take 2 large cups of dried lentils, rinse in cold water, and put them in a pressure cooker with enough water to cover them.
Cook the lentils in a pressure cooker for 5 minutes after the pressure cooker whistles, no more no less; then drain the lentils and grind them in the meat grinder until you get a rough mixture, without any whole lentils.
At this point, mix the chopped lentils with the egg and the remaining ingredients exactly as you would for meatballs, until you get a mixture that does not stick to your hands. If you prefer firm and compact meatballs, keep the recipe unchanged; but if you like softer meatballs, replace the breadcrumbs with some stale bread soaked in water.
Prepare a simple tomato sauce in a saucepan (if you have some already prepared, that's fine too) and dilute it with a little hot water; in the meantime, form the meatballs, taking care to make them rather small because they will grow a lot during cooking.
When they are ready, transfer the meatballs to the simmering sauce, and cook until the sauce is very dry, adding water if instead you feel that it needs a few more minutes of cooking.