For the bread: Place an oven rack in the center of the oven. Preheat the oven to 400 degrees F.
Arrange the bread halves on a baking sheet and drizzle with olive oil. Bake until light golden, 13 to 15 minutes. Cool for 2 minutes. Rub the warm bread with the cut-side of the garlic. Set aside.
For the topping: While the bread is cooking, in a large skillet, heat the oil over medium-high heat. Add the garlic and shallots and cook, stirring frequently, until soft, about 2 minutes. Sprinkle the shrimp with salt and pepper and add to the skillet. Cook for 3 to 4 minutes until just pink and cooked through. Remove the shrimp to a cutting board.
In the same skillet, add the tomatoes and season with salt and pepper. Cook over medium-high heat until the tomatoes start to soften, about 4 minutes.
Meanwhile, cut the shrimp into 1/2-inch pieces.
Turn the heat under the skillet to high. Add the wine and, using a wooden spoon, scrape up the browned bits that cling to the bottom of the pan. Boil for 2 minutes. Add the stock and boil until most of the liquid has evaporated, about 2 minutes.
Remove the pan from the heat. Add the shrimp, arugula, mascarpone cheese and tarragon, and stir until the mixture is creamy. Season with salt and pepper. Spoon the creamy shrimp and sauce over the toasts and sprinkle with salt and pepper. Cut into slices and serve.