Preheat oven to 350°F. Prepare mini muffin pan with vegetable pan spray.
For crust, combine flour, 3 tablespoons finely chopped thyme and salt in a small bowl; set aside. In a large bowl, beat butter and 1/3 cup granulated sugar with electric mixer until smooth. Beat in 1 tablespoon zest, egg yolk, vanilla and water. Add flour mixture; beat until just combined. Divide dough into two disks. Wrap each disk in plastic wrap and refrigerate for about 30 minutes.
For whipped cream, bring heavy cream to a simmer in a small saucepan; do not boil. Remove from heat and add 4 thyme sprigs. Let thyme steep 30 minutes. Remove thyme and cool completely in refrigerator, about 30 minutes.
In a small bowl, combine 1/2 cup diced grapefruit, remaining 1 tablespoon granulated sugar and 1 tablespoon finely chopped thyme. Cover and refrigerate, stirring occasionally, until fruit is soft; about 1 hour.
On lightly dusted surface, roll out one disk of dough until it is 1/8 in. thick. Using 3 in. scalloped cookie cutter cut out 12 rounds; re-rolling dough if necessary. Lay rounds in cavities of the muffin pan and gently press up sides. Prick dough with fork. Repeat with remaining disk of dough.
Bake 11-13 minutes or until slightly brown around edges. Remove pan to cooling grid and cool completely. Remove cooled crusts from pan.
In a medium bowl, combine lemon curd, remaining 1/4 cup diced grapefruit and remaining 1 tablespoon zest; stir to combine. Refrigerate until ready to use.
In a large bowl, combine chilled cream and confectioners' sugar. Whip until cream holds soft peaks.
Spoon 1 tablespoon of filling into each tartlet. Pipe or spoon whipped cream over filling and top with drained diced grapefruit.