1 Coarsely chop the onion. Slice the mushrooms.
2 Bring a large pot of water to a boil.
3 In a large skillet, warm the oil over medium-high heat until hot but not smoking. Add the onion and garlic and cook until the mixture is golden, about 5 minutes.
4 Add the butter and warm until melted. Reduce the heat to medium, add the mushrooms and cook until the mushrooms are wilted, about 5 minutes.
5 Add the pasta to the boiling water and cook until al dente, 10 to 12 minutes, or according to package directions.
6 Meanwhile, squeeze the thawed spinach with your hands to remove any excess moisture.
7 Add the spinach and basil to the skillet and cook, stirring, until heated through, about 1 minute.
8 Stir in the sour cream, Parmesan, salt and pepper, and cook, stirring, just until heated through, about 1 minute.
9 Drain the pasta and top with the sauce.