Prepare the dough by combining the flour, ricotta, sugar, baking powder, and salt in a bowl. Stir in the milk, oil, and egg until you have a workable dough. If it's too dry, add a little more olive oil or milk, a tablespoon at a time. Roll the dough into a ball and let it rest while you prepare the plums.
Roll out the dough to about an inch high to fit your chosen, well-buttered pan (I used a 26-centimeter/10-inch ceramic pie dish, but you can also do this as a rectangular cake, like a sheet cake). Lay the dough in the pan, pushing gently with fingers to even the surface. Lightly press the plum halves, cut side down, into the dough. Bake at 390º F (200º C) for 30 minutes or until the plums are soft and cooked and the top golden and springy.
The Australian-Japanese cookbook author has lived in Florence (where a visit to a cheese farm once inspired her to start a food blog) for over 10 years with her Tuscan sommelier husband and two kids. Her third cookbook, Tortellini at Midnight, is out now.