Sauté onion in 2 tablespoons olive oil in a skillet over medium heat until onion is translucent, about 5 minutes. Stir garlic into onion and turn off heat. Transfer onion mixture to a large mixing bowl.

Stir ground beef, ricotta cheese, parsley, egg, salt, black pepper, and cayenne pepper with onion mixture until almost combined; stir in bread crumbs and continue to mix until thoroughly blended.

Roll about 2 tablespoons of mixture into a 1-inch ball for each meatball. Pour remaining 2 tablespoons olive oil in same skillet used to cook onions. Place skillet over medium heat and brown meatballs on all sides in hot oil, about 5 minutes. Hold a crumpled paper towel in tongs and use it to remove excess grease from skillet.

Pour marinara sauce and water over meatballs in skillet. Stir to combine and bring to a simmer. Reduce heat to medium-low and simmer, stirring occasionally, until meatballs are cooked through and no longer pink in the center, about 30 minutes.