Grate the zucchini and squeeze out excess moisture with your hands or a clean towel; set aside.
In a large mixing bowl combine ground beef, egg, breadcrumbs or panko, minced garlic, grated zucchini, parsley, oregano, dried basil, crushed red pepper, sea salt, and black pepper; mix until just combined.
Form the mixture into 1½–2 inch meatballs, about 18 meatballs total.
For stovetop method: pour one jar of marinara into a large pot, add the meatballs in a single layer, then pour the second jar over them. Simmer over low heat, covered, for about 30–35 minutes, stirring occasionally, until an instant-read thermometer reads 165°F in the center of the meatballs.
For oven method: place meatballs on a parchment-lined baking sheet and bake in a 400°F oven for 17–20 minutes or until cooked through and 165°F internal temperature.
Let meatballs rest a few minutes, blot any excess grease if needed, then serve with marinara.