In a large bowl, using an electric mixer, beat softened cream cheese, 2 tablespoons sugar, milk and vanilla, on medium speed until well combined. Spread the cream cheese mixture into the bottom of the pastry shell.
In a small bowl, mash 1/2 cup strawberries. Arrange the remaining strawberries halves over the cream cheese mixture.
In a small saucepan, combine mashed strawberries, rhubarb, 3/4 cup water, and 1/2 cup sugar; bring to a boil. Reduce heat; simmer 2 minutes or until the rhubarb is tender. Drain, reserving 1 cup juice. (If necessary, add enough water to equal 1 cup.)
Bring the juice and cornstarch to a boil over medium heat, stirring constantly. Boil 2 minutes or until the mixture is clear and thickened. Stir in strawberry-rhubarb mixture and a few drops of red food coloring, if desired. Cool slightly. Spoon over strawberries. Refrigerate until chilled and set.