Warm the butter in a large frying pan over medium heat, and sauté the apple cubes. Add the lemon juice, and stir for a couple of minutes. Sprinkle with a bit of lemon zest and some pepper. Remove the apples from the pan and set aside. Allow to come to room temperature before assembling the tartines.

In the same pan, warm the oil, and sauté the shrimp over medium-high heat, stirring frequently. When the shrimp are cooked, after a couple of minutes, add the remaining lemon zest and season with salt to taste. Allow to come to room temperature before assembling the tartines.

Assemble at the last minute, topping each slice of bread with the shrimp and apples.