Preheat oven to 350 degrees.  Process flour, sugar and salt together in a food processor until just combined. Scatter butter pieces over the top.  Process until mixture has a sandy texture, about 12 long pulses. Turn dough out into a 9 inch round tart pan with a removable bottom.

Press dough into an even layer over the bottom and up the sides of the tart pan creating a crust.  Flour the bottom of a metal measuring cup and press it over the bottom and sides of the crust to help even everything out.  Freeze tart shell for 15 minutes.

Prick the bottom of the crust several times with a fork.  Place tart pan on a baking sheet and bake until golden brown, about 25-30 minutes.  Check on the crust occasionally to ensure that it isn’t puffing up. If it is, gently press it down with a fork or wooden spoon.  Allow tart shell to cool completely in the tart pan.

Make the filling:  Combine cream cheese, skyr and sugar in a stand mixer fitted with a paddle attachment.  Mix on medium speed until filling is fully combined and slightly increased in volume, about 3 minutes.

Transfer filling to cooled tart shell (continue to keep the tart shell in the pan. The crust can be delicate and the pan will help support it while you are going through the process of filling it).  Even out the filling with an offset spatula.  

Warm jam for about 30 seconds in the microwave until it becomes liquid.  Strain through a fine mesh strainer. In a large bowl, gently toss mixed berries with strained jam.

Carefully spoon berries on top of tart filling.  Lightly cover with plastic wrap or invert a plastic container over the top of the tart and chill for at least an hour or up to 6 hours. Leave in pan until just before serving.