Heat oven to 400 degrees. Roll out pastry and line quiche or tart pan. Line with foil and weight with pastry weights or dry beans. Bake 10 minutes, remove foil and weights and bake until golden, about 15 minutes more.
While pastry bakes, heat butter in large skillet. Add leeks, season with pepper and sauté over low heat until soft but not brown. Remove from heat.
When pastry is done, remove from oven and lower oven heat to 375 degrees. Spread leeks in pastry. Add cheese and thyme. Mix eggs and half-and-half and pour over ingredients in pastry.
Bake tart 35 to 40 minutes, until filling feels fairly firm to the touch and has barely begun to brown. Serve warm or at room temperature.