Line a 9x9-inch square baking pan with aluminum foil. Butter the foil and set aside.
In a medium saucepan over low heat combine the white chocolate and sweetened condensed milk. Stir until the white chocolate is just melted.
Remove from heat and stir in lime zest and lime juice.
Evenly spread the mixture into the prepared 9x9-inch pan. Sprinkle the chopped crystallized ginger on top and gently press into the fudge. Chill for 2 hours or until firm.
Using the edges of the foil, gently lift the fudge from the pan and place on a cutting board. Use a knife to cut the fudge into bite-sized pieces.