1 package (375 grams) ground mild or spicy Italian sausage
1 cup finely diced white onion
1 teaspoon minced garlic
1 package (283 grams) frozen chopped spinach, defrosted and squeezed of excess moisture
1/2 cup dry Italian-seasoned bread crumbs
2 eggs, beaten
3/4 cup grated Parmesan cheese
1/2 teaspoon salt
Twenty-four 4 centimeter squares of mozzarella cheese chopped fresh flat-leaf parsley, sea salt, and pepper, for garnish (optional)
Preheat the oven to 350°F. Place a 25 × 40 centimeter wire rack on a 32 × 47 centimeter baking tray (or any wire rack that fits into your baking trays will do).
To prepare the tomatoes, carefully slice off the top of each one. Using a teaspoon, carefully scoop out the insides until the cavity is clear. Discard the tops and the insides. Sprinkle a little salt in each tomato shell and then place upside down on the prepared wire rack to drain.
To make the filling, in a large skillet over medium-high heat, sauté the Italian sausage and onion until the sausage has cooked through and the onion is soft and translucent. Add the garlic and cook for another minute. Drain any excess fat.
Add the spinach and fry for three to four minutes to get rid of any excess moisture. Transfer the contents to a large mixing bowl to cool.
Once the mixture is cool to the touch, mix in the bread crumbs, eggs, Parmesan cheese, and salt.
Stuff each tomato with the filling just to the top, then place it upright on the rack. If it doesn’t stand up, you can slice off a small part of the tomato bottom to make it flat.
Bake for 30 to 35 minutes or until the filling reaches a minimum temperature of 165°F. Top each tomato with a slice of mozzarella cheese, then return to the oven. Bake until the cheese has melted.
Serve hot. These can be stored in the fridge in an airtight container or wrapped in plastic wrap for two to three days, but they are best when eaten fresh.