If using whole lobsters, cook them in boiling water for 6-8 minutes until bright red. Remove from boiling water. Once the lobsters are cooked, plunge them into the ice water to stop the cooking process. Shuck the lobster meat from the shells.
Bring a pot of water to a boil and cook pasta.
As the pasta cooks, make the bechamel sauce. Pour lobster stock into a large sauté pan over medium heat and bring to a simmer. Add the reserved roux to pan and stir slowly until sauce begins to thicken. Whisk lemon mascarpone and heavy cream sauce until incorporated. Add lobster meat, herbs and juices to the pan and cook on low heat. Add peas and half of the grated Parmesan cheese. Gently fold the ingredients together as they warm.
Once the pasta is tender, remove from boiling water and toss with the sauce in the same pan. Cook on low heat for a few minutes, until sauce coats the pasta. Add salt and pepper to taste. Use tongs to transfer the mixture to two warm bowls. Arrange the pasta on the bottom and lobster on the top in each bowl. Spoon half of the sauce over the top of each dish. Garnish with cracker crumbs (optional), remaining Parmesan cheese and chopped basil. Serve hot.
