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.
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