Heat 1 tablespoon olive oil in a large skillet or saucepan. Add the chopped onion with a pinch of sea salt and cook over medium heat for about 10 minutes, until the onion is translucent and slightly soft.

Add the minced garlic and dried oregano, and cook for another minute until the garlic is fragrant.

Pour in the two cans of tomatoes, and add about ¼ cup of water to each can. Swirl to get the remaining tomato bits, then pour that into the pan.

Stir in balsamic vinegar, sea salt, and cracked black pepper. Lower the heat and let the sauce simmer for about 15 minutes, stirring occasionally, until the sauce has thickened slightly.

Heat ½ tablespoon olive oil in a skillet. Add half of the spinach, and stir until it wilts. Add the remaining spinach and continue stirring until all the spinach is wilted.

Remove the spinach from the pan and let it cool slightly. Once cooled, squeeze out as much liquid as possible, then roughly chop the spinach and transfer it to a large mixing bowl.

Cook the pasta shells in a large pot of well-salted water until al dente, then drain and rinse under cold water to stop the cooking process.

In the bowl with the chopped spinach, add the ricotta, grated mozzarella, minced garlic, chopped basil, and season with salt and freshly ground black pepper. Mix well until combined.

Preheat your oven to 356°F (180°C).

Stuff each cooked pasta shell with about 2 teaspoons of the spinach and ricotta mixture. Be careful not to overfill.

Spread the prepared tomato sauce evenly in a baking dish. Arrange the stuffed shells in the sauce.

Sprinkle grated mozzarella and a little grated parmesan on top of the shells.

Bake the shells uncovered for about 20-25 minutes, or until the cheese is melted and bubbly, and the dish is heated through.