Whisk together the condensed milk and heavy cream in a large bowl until combined. Add the lime zest and juice and keep whisking about one minute until thickened.
Assemble the pie by spreading about 1/2 cup of the filling on the base of the serving dish (I used a 9 inch pie pan, but you can also use an 8x8 9inch square pan. Top with a layer of Ritz crackers. Spread another 1/2 cup or so of filling, then add another layer of Ritz crackers. Keep going until your final layer of filling. (I had four layers of filling and three layers of crackers)
Place in the fridge for 2 hours or overnight. The longer it sits, the more cake-like the crackers become. When ready to serve, prepare the whipped cream topping.