Tagliatelle with Chive Oil and Cremini Mushrooms

2 pounds cremini mushrooms, small ones left whole, large ones cut into halves
1/2 cup extra-virgin olive oil
1 teaspoon red pepper flakes
coarse salt and fresh ground pepper
3/4 pound dried tagliatelle
2 large bunches chives, roughly chopped (about 2 cups)
3 tablespoons fresh lemon juice
1/2 cup finely grated Parmesan cheese

1. Heat oven to 475 degrees. Toss together mushrooms with 1/4 cup olive oil, 1/2 teaspoon pepper flakes, and 1/2 teaspoon coarse salt on a baking sheet. Spread mushrooms out in a single layer, and roast in oven, turning a few times, until browned, about 10 minutes.

2. Meanwhile, bring large pot of salted water to boil. Add tagliatelle and cook until al dente, 8-10 minutes. Reserve 1/2 cup cooking water. Drain pasta and transfer it to a large bowl.

3. Put remaining 1/4 cup olive oil, 1/2 teaspoon pepper flakes, 1 3/4 cups chives, lemon juice, 1/4 teaspoon salt, and several grinds of pepper into a blender. Puree until emulsified, about 30 seconds. Make the chive oil as close as possible to serving time as its greenness diminishes as it sits.

4. Transfer chive oil to bowl with tagliatelle, add mushrooms, reserved pasta water, and parmigiana. Toss well. Adjust salt and pepper to taste. Serve immediately, garnished with remaining 1/4 cup of chopped chives.