Parsnips, Carrots and Leeks Gratin

My very good friend, Beej, made this for a Thanksgiving dinner we attended. It was soooooooo amazingly good - one of those things I couldn't stop eating despite my intolerance for dairy.

2 tablespoons butter (I add 2 T of olive oil too so the butter won't burn)
2 - 3 shallots, finely sliced (this is my own addition)
2 pounds leeks (white and pale green parts only), halved lenthwise, rinsed, cut crosswise into 1-inch pieces (about 5 cups)
1½ pounds large carrots, peeled, cut diagonally into ¼-inch-thick slices
1½ pounds large parsnips, peeled, cut diagonally into ¼-inch slices
2½ cups whipping cream
2 teaspoons of poultry seasoning
2 teaspoons Dijon mustard
¾ teaspoon salt
½ teaspoon pepper
½ cup coarsely grated Parmesan cheese

- Position rack in top third of oven and preheat to 400°F. Butter 13x9x2-inch glass baking dish.
- Melt 2 tablespoons butter in heavy large skillet over medium-low heat. Add shallots and leeks and sauté until soft and beginning to color, about 15 minutes. Transfer to large bowl.
- Cook carrots and parsnips in large pot of boiling salted water until almost tender, about 3 minutes. Drain well; place in bowl with leeks.
- Whisk cream, sage, mustard, salt and pepper to blend in medium bowl. Pour cream mixture over vegetables and stir gently to combine.
- Transfer to prepared baking dish. Sprinkle Parmesan over. Cover dish with foil. (Can be prepared 6 hours ahead; refrigerate. Let stand at room temperature 30 minutes before continuing.)
- Bake gratin 30 minutes. Uncover and bake until vegetables are tender, top is golden brown and cream is thickened, about 30 minutes longer. Let stand 10 minutes. Serve hot.