1. Heat large skillet over medium heat. Add olive oil, shallots and garlic. Cook just until aromatic. Add fennnel, carrot, leek and tarragon sprig. Cook until vegetables begin to soften. Add wine and cook until reduced by one-fourth.
2. Add cream and butter, and cook until heated through.
3. Meanwhile, cook pasta in large pot of boiling salted water until al dente. Drain, reserving 1/2 cup pasta-cooking water to add to vegetable mixture as needed for desired consistency.
4. Combine still-warm pasta with vegetable mixture. Add pasta water if additional moisture is needed. Season with salt and pepper. To serve, place crab on top each serving of pasta, and garnish with parsley, if desired.