Open-Faced Meatball Sandwiches
Preheat oven to 400°F (200°C). In a large bowl, use hands to mix ground beef, egg, breadcrumbs, milk, oregano, salt and pepper until just combined. Roll into 2-inch (5 cm) balls; place on a baking sheet lined with parchment paper. Bake until browned and cooked through (instant-read thermometer inserted in centre of meatball reads 160°F/71°C), 20 to 25 min. Place on paper towels to drain.
Meanwhile, in saucepan over medium heat, bring pasta sauce and water to simmer. Add drained meatballs; simmer, partially covered, for 10 min. Divide meatballs and sauce among slices of Italian bread; top with Parmesan cheese (if using).