Preheat oven to 400 degrees F.
Slice off a thin slice from the bottom and top of the tomatoes, just so the tomatoes will stand up. Carefully remove the pulp and seeds from the tomatoes with a paring knife or a grapefruit spoon. Stand in a baking dish, drizzle with olive oil and season with salt and pepper. Tear up the tarragon sprigs and place in the baking dish. Roast for 10 to 15 minutes, or just until the tomatoes begin to get tender. Remove and set aside, keeping warm.
Season rock shrimp with salt and pepper. In a large skillet over high heat, saute rock shrimp in olive oil for 2 to 3 minutes, until most of the liquid released by the shrimp has evaporated. Add shallots and cook for 1 minute. Add the cream, bring to a boil and reduce heat to a simmer. Cook until reduced by half and sauce covers the back of a spoon, about 4 to 6 minutes. Add the mustard and tarragon and whisk thoroughly to combine, about 1 minute. Season with salt and pepper.
Fill each tomato shell with some of the shrimp mixture. Serve.