How to Make It
Thinly slice 1 cup of the spinach; set aside. Heat oil in a Dutch oven over medium. Add garlic; cook until lightly browned and fragrant, about 1 minute. Stir in remaining spinach; cook, stirring constantly, until wilted, about 2 minutes. Add 1/4 teaspoon each of the salt and pepper; cook, stirring often, 2 minutes. Drain well, and finely chop spinach mixture.
Stir together ground turkey, sausage, breadcrumbs, onion, Parmesan cheese, eggs, chopped spinach mixture, and remaining 1/4 teaspoon each salt and pepper in a large bowl until well blended. Gently shape mixture into 20 (1-inch) balls.
Bring marinara sauce and water to a boil in a large skillet over medium, stirring occasionally. Reduce heat to medium-low, and maintain at a simmer. Add meatballs, and cook until meatballs are cooked through, 6 to 8 minutes. Reduce heat to low; cover and cook 10 minutes.
Remove from heat; sprinkle with provolone cheese. Let stand, covered, until cheese melts, 1 to 2 minutes. Divide meatballs and sauce among hoagie rolls. Top evenly with reserved sliced spinach. Serve immediately.
Mild fresh spinach can be slipped into just about any recipe. Simply wilt it in a pan with a little water or oil until it cooks down. Let the spinach cool; then wring out any excess moisture. Add it to pastas, top a pizza, or whisk it into scrambled eggs.