Preheat your oven to 375°F (190°C).
Cook the jumbo pasta shells in a large pot of salted boiling water according to package directions until al dente, then drain and set aside to cool.
Heat the extra virgin olive oil in a small skillet over medium heat and sauté the fresh baby spinach until completely wilted.
Remove the spinach from the skillet, squeeze out any excess moisture using a clean kitchen towel, and finely chop it.
In a medium mixing bowl, combine the shredded cooked chicken breast, part-skim ricotta cheese, chopped spinach, garlic powder, dried oregano, sea salt, and black pepper until well incorporated.
Spread roughly half of the marinara sauce across the bottom of a small oven-safe baking dish.
Stuff each cooked pasta shell with a generous portion of the chicken and ricotta mixture and place them open-side up in the baking dish.
Spoon the remaining marinara sauce over the stuffed shells.
Cover the dish with foil and bake for 15 minutes, then remove the foil and bake for an additional 5 minutes until the sauce is bubbling.
Garnish with freshly chopped basil before serving warm.