Bring a large pot of salted water to boil. Cook the pasta in the water until pasta is al dente, according to package directions. Gather the rest of your ingredients (even the spices) and make sure they are measured and on hand. Once the pasta has finished cooking, reserve about 1 cup of the pasta water. Drain the pasta and set aside.

Meanwhile heat a large, oven-safe skillet over medium high. Add the olive oil and butter and cook until the butter with canola oil melts. Add the mushrooms. Cook until the mushrooms begin to lightly brown, about 5 minutes, stirring every few minutes. Stir in the rosemary and salt. Sprinkle the flour over the top, then cook and stir for 1 minute, until the white bits disappear. The pan will seem dry.

Slowly pour in the milk, stirring as you go. Bring to a simmer. Stir in the pepper, garlic powder, and nutmeg. Let simmer until very slightly thickened, about 2 minutes. Stir in the cheddar and fontina until the cheeses melt, then stir in the drained pasta. If the sauce is too thick, splash in a little bit of the reserved pasta water. It should be rich, creamy, and nicely coat the noodles. Use the back of a spoon or spatula to spread the noodles into an even layer in the skillet.