Brush baguette slices with 2 tablespoons olive oil. Grill or toast bread slices until golden; arrange on a platter. Set aside.
Sauté garlic and shallots in remaining 6 tablespoons hot olive oil in a large skillet over medium-high heat 1 minute or until light golden brown. Add shrimp, and sauté 2 to 3 minutes or until shrimp turn bright pink. Turn shrimp, and cook 1 additional minute. Remove shrimp with a slotted spoon.
Add lemon juice and limoncello to skillet; cook over high heat until mixture is reduced by two-thirds, about 2 minutes. Return shrimp to mixture, stirring until shrimp are coated. Remove from heat; add salt and pepper to taste.
Spoon 1 shrimp with sauce onto each toast. Garnish, if desired.