Drain the ricotta cheese by placing it in a muslin cloth and hanging it for about 45 minutes to remove excess water.
Heat a pan with 3 tbsp olive oil. Add the chopped shallot and minced garlic, and sauté until soft and translucent.
Add the chopped spinach and salt. Cook until the spinach wilts and all moisture evaporates.
In a large bowl, combine the drained ricotta, spinach mixture, 3 cups mozzarella cheese, parmesan cheese, parsley, black pepper, Italian mixed herbs, and salt. Mix well.
Cook the jumbo pasta shells according to package instructions, along with needed salt, until slightly undercooked (al dente).
Drain the shells and transfer to a plate. Drizzle with remaining olive oil and gently toss to prevent sticking.
Preheat the oven to 375°F (190°C).
Fill each pasta shell with the prepared ricotta mixture using a spoon or piping bag.
Spread a layer of marinara sauce at the base of a baking tray. Arrange the stuffed shells over the sauce.
Pour the remaining marinara sauce evenly over the shells.
Sprinkle the remaining mozzarella cheese on top and garnish with parsley.
Cover the tray with aluminum foil and bake for 30 minutes.
Remove the foil and broil on high for 3–4 minutes until the top is golden and bubbly.
Remove from oven, let it rest for a few minutes, and serve warm.