• Filling can be made 1 day ahead and chilled, covered. Bring to room temperature before using. • Pissaladière strips can be baked 3 hours ahead and kept, uncovered, at room temperature.
Whisk together flour, baking powder, and salt in a bowl. Blend in butter with a pastry blender or your fingertips until most of mixture resembles coarse meal with some small (roughly pea-size) butter lumps. Add milk and stir with a fork until a shaggy dough forms. Gently knead dough 8 to 10 times with floured hands on a lightly floured surface.
04Roll out dough into a 12-inch square (1/4 inch thick) on a lightly floured surface with a floured rolling pin. Cut into 3 equal strips. Arrange strips 1 inch apart on an ungreased baking sheet and spread with filling, leaving a 1/2-inch border around edges.
05Bake until crust is golden, 20 to 25 minutes. Transfer to a rack using a large metal spatula and cool to warm or room temperature. Cut crosswise into 1-inch pieces.