Your first step is to zest about 3 - 4 key limes as you will need 2 teaspoons of zest. Then, juice 8 -10 key limes to get 1/4 fresh key lime juice from them. Set both aside.
Next, take the phyllo cups from the freezer so they thaw a bit before filling them.
Using a hand held mixer, beat the whipping cream in a large bowl, until soft peaks are formed. Set the bowl aside.
In another large bowl, beat the softened cream cheese, sweetened condensed milk, key lime juice, and zest until smooth and creamy. At this point, grab a rubber spatula and gently fold in he whipping cream into the lime cream cheese mixture.
Chill the key lime mousse filling for at least an hour before filling the mini tart shells.
Fill the mini phyllo shells right before serving so the bottoms of the shells do not get soggy. I like to spoon the chilled filling into a large resealable bag, then snip a small piece from one corner and pipe the filling into the phyllo shells. Garnish the tarts with a small slice of key lime and a fresh raspberry, if desired.
The key lime mousse filling can be made up to 2 days in advance and stored in the refrigerator. Then give it a good mix and fill the mini tart shells.