In a large nonstick skillet, heat the olive oil over MEDIUM-LOW heat. Add the onion and cook, stirring occasionally, for 2 to 3 minutes, or until tender (reduce heat if onion begins to burn). Add the garlic and cook about 30 seconds more. Remove the skillet from the heat and set it aside.

In a large bowl, whisk the egg, then add the bread crumbs, ⅓ cup marinara sauce, Italian seasoning, salt and pepper and mix to combine. Crumble the beef over the mixture and then scrape the onions from the skillet into the bowl. Mix well and shape into six oblong patties - mine were about 4-inch long and 1-inch thick.

Preheat oven to 375 degrees F.

Place the same skillet (with oil residue), over MEDIUM-HIGH heat. If the pan is dry, add a little additional oil and when hot, add half the patties, leaving a little space in between. Cook for several minutes on each side, or until nicely browned and partially cooked through. Transfer the browned, partially cooked patties to a 13-x 9-inch baking dish and repeat to brown the remaining patties. Top each patty neatly with the remaining ⅔ cup marinara sauce and the shredded mozzarella.

Transfer to the preheated oven and bake for 15 minutes or until the cheese has melted and the beef patties are cooked through to an internal temperature of at least 165 degrees F. Remove from the oven and garnish with fresh basil or parsley, if using. Let them rest for 5 minutes before serving.