In a large sauté pan, heat 2 tablespoons of the oil. Add the mushrooms, then season with salt and pepper. Cook over medium heat, stirring, until the mushrooms have given up their liquid and are slightly caramelized (about 7 to 10 minutes).
In a small sauté pan, cook the onions in the remaining 1 tablespoon of oil until they turn slightly translucent. Add the garlic and stir until the garlic is soft but not brown.
Add the onion mixture to the mushrooms. Add the sherry and chicken stock and reduce by about half. (Watch it! This can go fast. If the liquid reduces too much, you won’t have enough sauce to coat the pasta). Add the cream, rosemary, and thyme; reduce somewhat until the cream has thickened. (Take a big whiff of the combination of the flavorful liquids, mushrooms, and herbs—it’s pretty awesome.)
Meanwhile, cook the pasta according to package instructions, undercooking it just a little. Drain, then add it to the mushroom mixture. Stir well until the pasta is evenly coated and has soaked up most of the flavorful liquid. Add the parsley and chopped tomato, stir just to combine, and taste for seasoning. Serve in warmed bowls, with freshly grated parm on the table.