In a large bowl, with a mixer on high speed, beat butter and 1 cup sugar until smooth. Add eggs one at a time, beating well after each addition. Beat in lemon peel.
In a small bowl, mix flour, baking powder, salt, and nutmeg. Stir half the flour mixture into the butter mixture, then add the milk, followed by the remaining flour mixture; stir just until incorporated. Scrape batter into a buttered and floured 8-inch square glass or ceramic baking pan and spread level. Overlap plum slices in rows or concentric circles over batter. In a small bowl, mix remaining 2 tablespoons sugar and the cinnamon; sprinkle evenly over plums.
Bake in a 325° regular or convection oven until a wooden skewer inserted in the center comes out clean, 45 to 50 minutes. Serve warm or at room temperature.
This was delightful! I had it in a convection oven at 325 and it was still runny. I turned it up to 350 and let it go another 10 minutes. It came out perfect! I definately will make this again. The plums were great on top, but I think I'm going to experiment with other fruits too.
The flavor of this buckle was very good. My house smelled fabulous while it was baking. I had the same problem that the first reviewer faced. But, I figured out why. My plums were very ripe and juicy. It was still runny after 50 minutes. So, I switched to convection bake, raised the temp to 350, and baked for another 15 minutes. Voila...a beautiful, moist, and crispy topped buckle.