Bake the tart shells on a parchment paper-lined baking sheet as per package directions, and cool. Place the cream and garlic in a tall pot. Bring to a simmer and simmer until the cream is thickened and reduced to 1 cup. Remove from the heat and stir in the shrimp, crab, chopped dill or tarragon, and pepper. Cool to room temperature. Remove tarts from their foil liners and set back on the baking sheet. (If you wish to serve the tarts later, cover and store the seafood mixture in the fridge. Cover the tart shells and leave out at room temperature until ready to stuff and bake.) Preheat the oven to 350 degrees F. Fill the tarts with the seafood mixture; sprinkle with the cheese. Bake for 10 minutes, or until the seafood is well heated through. Garnish with dill or tarragon sprigs, if desired.