1. Combine sour cream and peas in a small bowl.
2. Cook the pasta according to package directions. Drain, reserving 1/4 cup of the pasta water.
3. Return pasta to pot and fold in sour cream mixture, ham, 3/4 cup cheese, tarragon, salt, and pepper.
4. Divide among 4 bowls and garnish with additional tarragon and remaining Parmesan cheese.