Preheat oven to 350 degrees. Grease a 10-inch tart pan or 9-inch spring-form pan.
For the pastry: In a bowl, cream together butter and sugar. Beat in egg. Slowly mix in flour until dough comes together. If dough appears too dry, add a little milk. Form into a ball; wrap in plastic and chill for 30 minutes.
Meanwhile, make the filling: Beat together the eggs and sugar with an electric mixer on medium-high speed for 3-4 minutes or until creamy, lightened in color, and doubled in size. Gently fold in the lemon zest, ground almonds and cinnamon.
On a floured work surface, roll out pastry dough until it is about 1 inch larger than the pan on all sides. Place pastry into pan, trimming edges as needed so the pastry comes about 1 inch up the sides of the pan. Prick all over with a fork and spoon in filling, smoothing the top with a spatula. Bake for about 30 minutes, or until golden brown.
Allow tart to cool in the pan. Once cool, transfer to a serving plate and dust with powdered sugar before serving.