In large frying pan on medium heat add olive oil and butter.
Add sliced mushrooms, chopped onions, thyme and garlic and saute until soft and lightly brown.
Sprinkle with salt.
Turn down to a simmer.
In a small saucepan melt the 1/4 cup butter.
On low heat add the arrowroot starch and whisk.
Add the milk slowly whisking until sauce is smooth.
Increase heat to low-medium and whisk until sauce thickens.
Turn to low and add shredded parmesan cheese.
Pour white sauce over sautéed mushrooms and stir.
Keep sauce on low heat until you are ready to serve.
In a large stockpot make the pasta according to package directions.
To serve place cooked pasta in individual bowls and top with the hot mushroom sauce.
Sprinkle shredded parmesan on each dish.