Cook pasta according to instructions on the box. Drain, keep warm.
In a large saucepan, melt 2 tablespoons of butter.
Add mushrooms and garlic and sautee for 2 to 3 minutes, until cooked.
In a medium mixing bowl, whisk together half and half, milk and cornstarch.
Add the milk mixture to the saucepan, about ¼ of a cup at a time, whisking after each addition until the sauce thickens (add ¼ c to the saucepan, stir until thickens, add another ¼ of a cup and stir until thickens, repeat). After you added all the milk mixture, cook the sauce stirring for 1 more minute. Add the remaining tablespoon of butter and stir until melted. Cool slightly, taste and season with salt and pepper as needed.
Add pasta to the sauce and gently stir together.
Serve, garnished with chopped cilantro and slices of focaccia.
Notes
This recipe makes 2 cups of sauce that can be used for other dishes, like baked pasta.