To make pastry, mix flour and chilled diced butter in food processor until butter is rubbed in. Add sour cream and combined. Refrigerate for 10 minutes.
Meanwhile, prepare leek filling by bringing salted water to boil in medium saucepan. Wash leek and cut into quarters and finely slice. Rewash, add to water and blanch, then remove from heat. Drain and put in iced water. Once cooled, drain water and squeeze off through a tea towel and set aside. Mix yogurt, egg, salt and nutmeg together, add leek.
For sticky mushrooms, wash mushrooms and dry. Heat olive oil in heavy based saucepan, add mushrooms and salt, sweat with lid on. Add red wine and reduce on low heat by half. Then, add sugar and continue to reduce mixture until sticky.
To make tart cases, preheat oven to 180°C. Roll out pastry on floured bench and cut 4 rounds. Dust off and flour place in oiled tart cases. Refrigerate 5 minutes. Blind bake for approximately 10 minutes. Remove and allow to cool. Remove from tart tins. Add leek mix and bake for approximately 8 minutes.
Trim beef and cut into 180g portions. Brush with mustard and roll in seasoned rice flour. Preheat olive oil in medium pan and seal steaks. Finish in oven for approximately 8 minutes; remove from oven and allow to rest 3 to 5 minutes.
Heat beef glaze and roughly chop sage and thyme for garnishing.
Place tart cases onto serving plates and fill with beef portions. Surround each pastry case with beef glaze and sticky mushrooms. Garnish with cut sage and thyme.