Step 1Heat a medium, heavy-bottom pot over medium-high heat until hot. Add one-half cup (1 stick) of butter, swirling to melt.
Step 2Add the button mushrooms and saute until lightly colored, 2 to 3 minutes. Add the spinach and continue to saute for a minute to wilt.
Step 3Stir in the heavy cream, smoked Gouda, 8 ounces pecorino cheese and truffle oil, and reduce the heat slightly. Continue to cook, stirring frequently, until the sauce starts to thicken, about 5 minutes.
Step 4Toss in the pasta and heat through, stirring until the sauce is thickened and creamy. Taste and season if desired. Remove from heat and set aside in a warm place.
Step 5Heat a saute pan over medium-high heat until hot. Melt the remaining butter and saute the mixed mushrooms until lightly browned, 2 to 3 minutes. Remove from heat.
Step 6In a small bowl, toss together the panko crumbs with the remaining 1/4 cup Parmesan cheese.
Step 7To plate, divide the pasta among 4 to 8 oven-proof bowls. Top the pasta with the bread crumb mixture and place the bowls under the broiler. Broil the pasta just until the topping starts to brown, about a minute, depending on the distance of the broiler.
Step 8Remove the bowls from the broiler and top each serving with the sauteed mushrooms and a sprig of parsley. Serve immediately.