Follow these steps carefully for perfectly tender peppers and a flavorful, creamy filling:

1. Preheat Oven & Prepare Baking Dish: Preheat your oven to 375°F (190°C). Lightly grease a baking dish large enough to hold the pepper halves snugly in a single layer (a 9×13 inch dish often works well). If using marinara sauce, spread it evenly over the bottom of the dish. This adds moisture and prevents the peppers from sticking.
2. Prepare the Bell Peppers: Wash the bell peppers thoroughly. Slice them in half lengthwise, from stem to bottom. Carefully remove the seeds, ribs, and stems. You want a clean, hollowed-out pepper half ready for stuffing. Some prefer to trim a tiny sliver off the rounded bottom of each half so they sit flat in the dish, but be careful not to create a hole.
3. Par-Cook the Peppers (Optional but Recommended): Place the pepper halves, cut-side down, in the prepared baking dish. Add about ¼ inch of water to the bottom of the dish (skip if using marinara sauce). Cover the dish tightly with foil. Bake for 15-20 minutes. This step helps to soften the peppers, ensuring they are tender after the final bake. If you prefer firmer peppers, you can skip this step. Remove the peppers from the oven, carefully drain any water, and set aside. Keep the oven preheated.
4. Sauté Aromatics: While the peppers are par-cooking (or while the oven preheats if skipping step 3), heat the olive oil in a large skillet or sauté pan over medium heat. Add the finely chopped onion and cook, stirring occasionally, until softened and translucent, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant, being careful not to burn it.
5. Incorporate Spinach: Add the thawed and thoroughly squeezed dry chopped spinach to the skillet with the onions and garlic. Stir well to combine and cook for 2-3 minutes, just to heat the spinach through and allow any remaining moisture to evaporate. Remove the skillet from the heat.
6. Prepare the Ricotta Filling: In a large mixing bowl, combine the whole milk ricotta cheese, the lightly beaten egg, ½ cup of grated Parmesan cheese, the optional breadcrumbs, dried Italian seasoning, salt, and black pepper. Stir everything together until well combined.
7. Combine Filling Ingredients: Add the cooked spinach, onion, and garlic mixture from the skillet to the ricotta mixture in the bowl. Stir gently but thoroughly until everything is evenly distributed. Taste the filling and adjust seasoning (salt, pepper, herbs) if needed. Remember that the Parmesan adds saltiness.
8. Stuff the Peppers: Arrange the par-cooked (or raw) pepper halves, cut-side up, in the baking dish (over the optional marinara sauce, if using). Generously spoon the spinach and ricotta filling into each pepper half, mounding it slightly. Don’t pack it too tightly, but ensure each pepper is well-filled.
9. Top and Bake: Sprinkle the tops of the filled peppers generously with the extra grated Parmesan cheese. This will create a lovely golden-brown crust.
10. Bake Until Tender and Golden: Carefully place the baking dish back into the preheated 375°F (190°C) oven. Bake uncovered for 30-40 minutes (or potentially 45-55 minutes if you skipped the par-baking step). The peppers should be tender when pierced with a fork, the filling should be heated through and set, and the Parmesan topping should be melted and nicely golden brown. If the tops start browning too quickly, you can loosely tent the dish with foil for the remainder of the baking time.
11. Rest Before Serving: Once baked, remove the stuffed peppers from the oven. Let them rest in the baking dish for 5-10 minutes before serving. This allows the filling to set further and makes them easier to handle. The flavors also meld beautifully during this resting period.
12. Serve: Garnish with fresh chopped parsley or basil, if desired, and serve warm.