In a small bowl, mix mascarpone and lemon zest. Spread a thin layer of the mixture on 1 side of each baguette slice and set on a platter (save any leftover cheese mixture for another use).
Pour oil into a large frying pan over medium-high heat. Add garlic and cook, stirring often, until fragrant but not beginning to brown, about 1 minute. Add shrimp and cook, stirring often, until beginning to turn pink on the outside, 2 to 3 minutes. Increase heat to high, pour in wine, and boil, stirring often, until liquid is almost all evaporated and shrimp are opaque but still moist-looking in center of thickest part (cut to test), about 4 minutes. Stir in lemon juice, chopped tarragon, and pepper to taste. Remove from heat.
Spoon a couple of shrimp with some sauce onto each toast and sprinkle with sea salt. Garnish each with a tarragon sprig.
Note: Nutritional analysis is per crostini.