Preheat oven to 350°. Line bottom of a greased 9-in. springform pan with parchment; grease parchment. In a small saucepan, mix cornstarch and sugar; stir in strawberries, rhubarb and water. Bring to a boil. Reduce heat; simmer, uncovered, until thickened, 6-8 minutes, stirring occasionally. For filling, in a small bowl, beat cream cheese and egg until smooth.
In a large bowl, combine flour and sugar; cut in butter until crumbly. Reserve 3/4 cup for topping. Stir baking powder, baking soda and salt into remaining flour mixture. In a small bowl, whisk egg, sour cream and vanilla until blended; gently stir into flour mixture (do not overmix).
Spread batter onto bottom and 1/2 in. up sides of prepared pan. Spread filling over crust, leaving a 1/2-in. border around edge of pan. Spoon strawberry mixture over top; sprinkle with reserved crumb mixture.
Bake until edges are golden brown, 50-60 minutes. Cool on a wire rack 20 minutes. Loosen sides from pan with a knife. Cool completely. Remove rim from pan. Refrigerate leftovers.