Preheat your oven to 350°F and lightly grease a 9x13 inch baking dish. While the oven heats, prepare all your ingredients: finely dice the onion into 1/4-inch pieces, dice the green bell pepper, mince the garlic, and shred the mozzarella cheese from a block (freshly shredded melts much better than pre-shredded). Having everything ready before you start cooking ensures the workflow stays smooth and nothing burns while you're prepping.

In a large skillet over medium-high heat, add the ground beef, ground Italian sausage, diced onion, and green bell pepper. Cook for 8-10 minutes, breaking up the meat with a spoon as it cooks, until the meat is completely browned and the vegetables are softened. Drain off excess fat, then add half the minced garlic (about 3 cloves) and cook for 20 seconds until fragrant. Stir in the marinara sauce and Italian seasoning, then reduce heat to low and let it simmer gently while you prepare the alfredo sauce. I prefer Rao's marinara because the slow-simmered flavor really shines through in the finished dish.

While the meat sauce simmers, bring a large pot of salted water to a boil and add the spaghetti, cooking until just al dente according to package directions (usually 9-11 minutes)—don't overcook it since it will continue cooking in the oven. When the pasta is nearly done, start making the alfredo in a separate medium saucepan: melt the butter over medium heat, then add the remaining minced garlic (about 3 cloves) and cook for just 20 seconds. Pour in the heavy cream and bring to a gentle simmer, stirring occasionally, for about 2 minutes until it slightly reduces. Using Kerrygold unsalted butter gives the sauce a beautiful richness that really elevates this dish.

Remove the cream sauce from heat and stir in the parmesan cheese until fully melted and smooth. Season with salt, pepper, and nutmeg, whisking until the sauce is creamy and well-blended. Taste and adjust seasonings if needed—the nutmeg might seem like an odd addition, but it adds a subtle warmth that makes the sauce taste like a restaurant-quality alfredo.

Drain the spaghetti thoroughly and return it to the pot (or a large bowl). Pour the alfredo sauce from Step 4 over the pasta and toss until every strand is well coated. Transfer this creamy pasta mixture to your prepared baking dish, spreading it in an even layer. Pour the meat sauce from Step 2 evenly over the top, then sprinkle the shredded mozzarella cheese in an even layer covering the entire surface.

Bake in the preheated 350°F oven for 20 minutes until the mozzarella is melted, bubbly around the edges, and the casserole is heated through. Let it rest for 2-3 minutes before serving—this helps everything set slightly so it holds together beautifully on the plate. Serve hot with crusty bread on the side to soak up the creamy sauce.