Cook pasta, gather ingredients: Place a 10-inch cast-iron skillet or other ovenproof skillet in the oven and preheat the oven to 375°F. Bring a pot of salted water to a boil and cook spaghetti according to package directions.

Cook sausage and peppers: While pasta cooks, heat 2 teaspoons oil in a large nonstick skillet over medium heat. Add the sausages and brown, breaking the pieces into small chunks with a spoon, until cooked through, about 6 to 8 minutes. Add the bell pepper and cook until softened, about 5 minutes. Transfer the sausage mixture to a medium mixing bowl, set aside.

Cook onions and garlic, add tomato sauce: Adding more oil to the skillet if needed, sauté the onion over medium-high heat until softened and translucent, about 4 minutes. Add the garlic and cook, stirring constantly, for 1 minute. Stir in the tomato sauce or puree and ½ teaspoon Italian seasoning. Bring to a simmer, and cook for 4 minutes to allow flavors to meld. Season with salt and ¼ teaspoon pepper, adding additional if desired. Remove from heat.

Combine spaghetti pie mixture: In a small bowl, mix the egg, ricotta, the remaining ¼ teaspoon black pepper, and remaining ½ teaspoon Italian seasoning until smooth. Add the cooked, drained spaghetti to the bowl with the sausage. Stir in the ricotta mixture, about half the tomato sauce (about 1 cup) and ½ cup mozzarella cheese. Use tongs to mix the spaghetti thoroughly with the other ingredients.

Add pasta to preheated pan and bake: Remove the hot cast-iron pan from the oven. Using a basting or pastry brush, brush the remaining 1 teaspoon oil over the bottom and sides of the pan. Immediately pour the spaghetti mixture into the pan, using a spoon or tongs to distribute it evenly. Smooth the remaining tomato sauce over the top of the spaghetti and sprinkle with the remaining ½ cup mozzarella cheese. Return the pan to the oven and bake for about 30 minutes, until the cheese is bubbly and slightly browned. Remove from the oven and let sit for about 5 minutes before cutting into wedges and serving.