Cover the sun-dried tomatoes with 1/2 cup boiling water and let stand about five minutes. Drain, reserving the soaking liquid.
Melt butter in a heavy skillet over medium heat and use it to sauté the shallots for about one minute, or until they're tender.
Add the mushrooms and tomatoes and continue to sauté for about 3 minutes, stirring almost constantly.
Stir in the milk, salt, and white pepper. Add the reserved soaking liquid and bring to a boil. Reduce the heat to low and simmer for about 12 minutes, or until the volume is reduced by about 1/4.
Cook the pasta according to instructions, drain and transfer to a large bowl.
Pour the sauce over the pasta and toss. Sprinkle with the Parmesan and scallions.