Cook the pasta according to the package instructions. Drain and set aside. While the pasta is cooking, combine the porcini mushrooms and hot water. Set aside.

Remove the stems from the portobello mushrooms. Scoop out and discard the gills, then chop the mushroom caps into 1-inch chunks. Heat the oil in a large skillet over medium heat. Add the shallots, garlic, and portobello mushrooms. Sauté until the mushrooms have softened and started to release their juices, 3-4 minutes.

While the vegetables are cooking, remove the dried mushrooms from the liquid (reserve the liquid). Finely chop them and add them to the pan. Add the brown rice flour and stir until the flour dissolves. Pour 1½ cups of the mushroom liquid into the pan. Bring to a simmer. Cover and let cook for 15 minutes. Stir in the milk (or sour cream) and cook for another 5 minutes.

Add the cooked pasta to the skillet and toss to combine. Season to taste with salt and pepper. If the sauce seems too thick, stir in 1-2 tablespoons of water.

Serve immediately garnished with parsley and red pepper flakes, if desired.