Preheat the broiler or grill.
In a 12 to 14-inch saute pan, heat the olive oil over medium heat until just smoking. Add the garlic and cook until it turns light brown. Add the shrimp and cook without turning for 2 to 3 minutes, until bright red. Turn the shrimp over and cook for 1 minute. Add the lemon juice, and limoncello, and cook until reduced to about 1/3, about 2 minutes. Season with salt and pepper. Remove from heat and set aside.
Grill or toast the bread and place on a platter. Spoon 1 shrimp with sauce onto each toast, sprinkle with lemon zest and parsley, and serve immediately.