Bring a large pot of salted water to a boil and add fettuccine, cooking until al dente (about 10 minutes)—taste it at the eight-minute mark because dried pasta varies wildly by brand. I learned this the hard way when overcooked noodles sabotaged an otherwise perfect sauce at a dinner party years ago.
While pasta cooks, heat butter and olive oil together in a large skillet over medium heat until the foam subsides and the oil shimmers. This combination prevents garlic from scorching because the olive oil has a lower smoke point—mixing them moderately distributes heat more evenly than butter alone.
Add diced onion to the skillet and sauté for three minutes until translucent and softening at the edges. The onion will release its sweetness, which balances the earthiness coming from the mushrooms in the next step.
Add minced garlic and cook for exactly one minute, stirring constantly so it coats the fat but doesn't brown. Burnt garlic tastes bitter and ruins the entire creamy mushroom pasta family summer recipe, so don't wander away here.
Add sliced mushrooms and dried thyme, stirring occasionally for eight to ten minutes until the mushrooms release their liquid and that liquid mostly evaporates. You'll see the pan go from wet to dry—this concentration builds the sauce foundation that prevents the finished dish from tasting watery.
Reduce heat to low and pour in heavy cream slowly while stirring constantly for two minutes. Low heat keeps the cream from breaking and separating, which happens when cream hits high heat suddenly—I test every batch by pouring it in inches above the pan so it incorporates gradually.
Add mozzarella and Parmesan cheese off the heat, stirring until fully melted into a cohesive sauce that coats the back of a spoon. This easy mushroom pasta kids recognize comes alive at this moment when the sauce turns glossy and unified.
Drain the cooked fettuccine and add it directly to the skillet with the sauce, tossing gently for one minute so each strand gets coated. Season with salt and black pepper, then garnish with fresh parsley just before serving to keep its color and bright flavor intact.