Cook the pasta in boiling, salted water until al dente. Drain, reserving a little pasta cooking water. In a 2-quart saucepan, heat the olive oil. Sauté fennel, shallots and garlic until tender. Add the shrimp, and cook 2-3 minutes.
Add the white wine, and simmer until the liquid is reduced by half. Add the peas and pea tendrils and mascarpone. Salt and pepper to taste.
Add the pasta and toss thoroughly. You may need to adjust the sauce consistency by adding a little of the pasta cooking water. Garnish with fresh tarragon leaves.