Au Gratin Potato Tarts

1. Cut butter in dices, mix with flour and salt; blend the ingredients with your fingertips to obtain a sandy consistency; add enough water to form smooth dough; make a ball and refrigerate for 30 minutes.  
2. Spread the dough and line tart molds; pinch them with a fork and bake until cooked and almost golden brown.
3. Prepare mashed potatoes according to the instructions on the package; let cool and then add butter and egg.
4. Preheat the oven at 355°F.
5. For the filling heat the olive oil in a pan and fry the onion, the red pepper and the asparagus during 4 minutes; remove from heat and add the sour cream and cheese; stir until all ingredients are well mixed and season with salt and pepper.
6. Pour this mix in the bottom of the tartlets, cover with mashed potatoes and bake during 15 minutes.

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