01 - Preheat oven to 375°F. Lightly brush a large baking dish with olive oil to prevent sticking.
02 - Cut each bell pepper in half lengthwise and carefully remove the seeds and membranes. Arrange the pepper halves cut-side up in the prepared baking dish.
03 - Heat 1 tablespoon of olive oil in a skillet over medium heat. Add the chopped onion and garlic, sautéing until translucent, about 3 minutes. Add the spinach and cook until just wilted. Remove from heat and let cool slightly.
04 - In a large mixing bowl, combine the ricotta cheese, Parmesan cheese, egg, basil, parsley, salt, and black pepper. Fold in the cooled spinach mixture until evenly incorporated.
05 - Spoon the ricotta filling evenly into each pepper half, mounding slightly. Sprinkle shredded mozzarella over the top of each stuffed pepper and drizzle with the remaining olive oil.
06 - Cover the baking dish tightly with aluminum foil and bake for 25 minutes. Remove the foil and continue baking for an additional 10 minutes, until the cheese is bubbly and golden brown on top.
07 - Remove from the oven and let the stuffed peppers rest for 5 minutes before serving to allow the filling to set.