Prepare the aromatics and mushrooms: In a skillet over medium heat, melt the butter with the olive oil. Add the chopped onion and cook 3–4 minutes, until it softens and turns translucent. Stir in the minced garlic and cook about 30 seconds, just until fragrant.
Sauté the mushrooms: Add the sliced mushrooms to the skillet with the onions and garlic. Sprinkle with a pinch of the salt and pepper. Cook 5–7 minutes, stirring now and then, until the mushrooms give off their liquid and start to brown lightly. This step deepens the flavor, much like the old stovetop casseroles our mothers used to make.
Load the slow cooker: Transfer the onion and mushroom mixture to the slow cooker. Add the dried thyme, dried parsley (if using), remaining salt and pepper, and the vegetable or chicken broth. Stir gently to combine.
Slow cook the base: Cover the slow cooker and cook on LOW for 3–4 hours or on HIGH for 1½–2 hours, until the mushrooms are very tender and the broth is well flavored.
Add the pasta: Stir in the uncooked pasta, making sure it is mostly submerged in the liquid. Cover and cook on HIGH for 25–40 minutes, checking and stirring once or twice, until the pasta is just tender. Cooking time will vary by pasta shape and slow cooker, so start checking around 20–25 minutes.
Finish with cream and cheeses: Once the pasta is tender, turn the slow cooker to LOW or WARM. Stir in the heavy cream, cubed cream cheese, and grated Parmesan. Keep stirring until the cream cheese melts and the sauce becomes smooth and creamy. If the cream cheese is very cold, cover and let it sit 5–10 minutes, then stir again.
Adjust thickness (optional): If you prefer a thicker sauce, whisk the cornstarch and cold water together in a small bowl to make a slurry. Stir the slurry into the pasta and sauce, cover, and cook on HIGH for another 5–10 minutes, until slightly thickened. If the sauce seems too thick, add a splash of broth or cream to loosen it.
Taste and serve: Taste and adjust seasoning with additional salt and pepper, if needed. Spoon the creamy mushroom pasta into bowls, sprinkle with extra Parmesan and chopped fresh parsley if you like, and serve warm while the sauce is silky and comforting.
Variations & Tips
This is the sort of recipe that happily adapts to what you have in the pantry, just like the casseroles our grandmothers stretched to feed one more farmhand. For extra protein, stir in 1–2 cups of cooked shredded chicken, browned ground beef, or sliced smoked sausage when you add the cream and cheeses. If you enjoy a heartier, almost stroganoff-style dish, use beef broth instead of vegetable broth and add a spoonful of Dijon mustard and a splash of Worcestershire sauce. You can easily tuck in more vegetables: a cup of frozen peas or mixed vegetables can be stirred in with the pasta during the last 15 minutes of cooking, or add a handful of baby spinach when you add the cream so it wilts gently. For a lighter version, use half-and-half instead of heavy cream and reduce the cream cheese to 2 ounces, relying more on Parmesan for flavor. If you’re cooking for someone who needs to avoid gluten, use gluten-free pasta and keep a close eye on the timing, as it can soften more quickly. And, in true Midwestern fashion, don’t be afraid to top each serving with buttered breadcrumbs or crushed crackers toasted in a skillet—an old-fashioned crunchy topping that makes this slow cooker pasta feel like the cozy casseroles many of us grew up on.
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