Using serrated knife, slice baguette at one centimeter thickness. Note: Only slice completely through loaf on every OTHER slice, producing pockets. Spread butter on outer sides of each piece.
In large skillet over medium-high heat, toast crostini, flipping when golden brown, about one to two minutes per side. Set aside.
In bowl, stir together mayonnaise, lemon zest, lemon juice, mustard and garlic. Add lobster and stir to combine. Stuff each crostini clamshell with lobster mixture. Top with desired toppings. Serve immediately.