1. Preheat your oven to 375°F (190°C).

2. Line a small baking pan measuring 7x10 inches (18x25 cm) with parchment paper or grease it lightly with butter.

3. Cut the plums into thin wedges and toss them with cornstarch until evenly coated.

4. In a large bowl, combine oats, granulated sugar, brown sugar, baking powder, salt, and cinnamon.

5. Add cold butter and use your fingers or a pastry cutter to blend it into the mixture until it looks like coarse crumbs.

6. Press this mixture firmly into the bottom of the prepared pan, making sure it's evenly distributed.

7. Arrange the plum wedges in a single layer over the base, leaving a small border around the edges.

8. In another bowl, whisk together milk, eggs, and honey until smooth.

9. Pour this liquid evenly over the plums and crust.

10. Scatter chopped nuts on top and gently press them down so they stick.

11. Bake for 35 to 40 minutes, until the top is golden brown and the center is set.

12. Remove from the oven and let cool in the pan for 10 minutes.

13. Cut into bars and serve warm. Be careful when cutting the bars, as the knife may be sharp.