Preheat your oven to 375 degrees Fahrenheit for roasting, as this temperature perfectly intensifies the natural sweetness of the butternut squash without overcooking it. In a 9×13 inch baking pan, combine the 4 cups of diced butternut squash and 1/2 small yellow onion diced, then drizzle with 2 tablespoons of olive oil and add a good pinch of salt to enhance flavors. Toss everything together thoroughly to ensure even coating, then slide the pan into the oven for about 30 minutes or until the squash becomes tender and caramelized, developing a deeper, sweeter profile that forms the heart of this creamy sauce.​

While the squash roasts, fill a large pot with water and set it on the stove to come to a boil, adding a generous pinch of salt to season the pasta as it cooks. This step harmonizes with the roasting time, so by the last 10 minutes of squash cooking, your water will be ready. Keep the pasta dry until the squash finishes, allowing all components to sync seamlessly for efficiency in your kitchen workflow.​

Once the squash emerges tender from the oven after 30 minutes, let it rest briefly for about 10 minutes to cool slightly and release steam. Drop the 1 lb of elbow pasta into the boiling salted water and cook according to package instructions until al dente, typically 8 to 10 minutes. Drain the pasta immediately and return it to the same large pot without rinsing, preserving the starchy coating that helps the sauce cling beautifully later.​

Transfer the roasted butternut squash and onions directly into a blender, adding 4 oz of cream cheese and 1 1/2 cups of veggie stock to create a velvety base. Start by blending with half the stock for control, then incorporate the rest as needed until the mixture achieves a smooth, thick, and creamy consistency, not too thin but pourable. This puree captures the roasted essence, providing natural creaminess without heavy cream while sneaking in nutrients kids often overlook.​

Pour the gorgeous squash puree over the drained pasta in the pot, stirring promptly to coat every noodle evenly while the mixture remains warm. Add about three-quarters of the shredded cheeses, including 4 oz Monterey Jack, 4 oz sharp cheddar, and 1/4 cup grated Parmesan, mixing vigorously until they melt into a gooey embrace. Taste and adjust salt levels now, as the cheese amplifies savoriness; this stovetop stage already yields a delicious mac ready to eat if skipping the bake.​

Spoon the cheesy pasta mixture back into the 9×13 inch baking pan used for roasting, spreading it evenly for uniform baking. Sprinkle the remaining quarter of the cheeses across the top to form a melty blanket that bubbles invitingly. This transfer reuses pan residues for added flavor depth, turning the dish into a cohesive, oven-ready casserole.​

Mix 1/4 cup breadcrumbs with 2 tablespoons melted butter until it resembles wet sand, creating a crispy contrast to the creamy interior. Scatter this mixture generously over the cheese-topped pasta by hand or spoon for even coverage. The butter ensures golden browning, elevating the dish from stovetop comfort to baked perfection with a satisfying crunch.​

Place the assembled pan back into the still-hot 375-degree oven for 10 to 30 minutes, watching for a lovely golden brown top and bubbly edges. The shorter time crisps the breadcrumbs quickly, while longer ensures thorough heating if needed; broil briefly at the end for extra char if desired. Remove when the cheese melts fully and aromas fill your kitchen, signaling readiness.