TORTELLINI GRATINATA WITH MUSHROOMS AND PARSNIP BéCHAMEL

Steps:

• Cook parsnips in medium saucepan of boiling salted water until very tender, about 20 minutes. Drain; reserve saucepan. Transfer parsnips to processor. Add 1 1/2 cups milk. Blend until smooth. With machine running, gradually add remaining 1 cup milk through feed tube. Add 3/4 cup Parmesan cheese and nutmeg; blend well. Return sauce to reserved pan. Simmer over low heat until reduced to 3 cups, whisking often, about 5 minutes. Season béchamel sauce to taste with salt and freshly ground black pepper.
• Melt 2 1/2 tablespoons butter in heavy large skillet over medium-high heat. Add mushrooms, garlic, and rosemary. Sauté until mushrooms are brown and tender, 6 to 7 minutes. Season to taste with salt and pepper. DO AHEAD: Béchamel sauce and mushrooms can be made 1 day ahead. Cool, cover separately, and chill. Rewarm béchamel sauce before using.
• Preheat oven to 400°F. Butter 12-cup- capacity baking dish. Cook pasta in large pot of boiling salted water until just tender but still firm to bite, stirring occasionally. Drain pasta; return to pot. Add remaining 2 tablespoons butter and toss to coat. Stir in mushroom mixture. Transfer pasta to prepared baking dish. Spread béchamel sauce over; sprinkle with Gorgonzola, then remaining 1/4 cup Parmesan.
• Bake pasta until heated through and sauce is bubbling, 18 to 20 minutes. If desired, broil until top is golden, about 2 minutes. Let stand 10 minutes and serve.