Preheat your oven to 350°F first.
Cut plums into quarters and remove pits.
Toss them with sugar, flour, and lemon zest.
Spread them in your baking dish evenly.
Mix flour, brown sugar, and cinnamon together.
Cut cold butter into small cubes.
Add butter to the flour mixture.
Use your fingers to create coarse crumbs.
Stir in the oats gently.
Sprinkle topping over the plums evenly.
Cover every bit of fruit completely.
Bake for 45 to 50 minutes.
The top should turn golden brown.
Let it cool for 10 minutes.