Preheat oven to 375°F (190°C). Grease an 8x8-inch (20x20 cm) baking dish.
In a large bowl, mix oats, baking powder, cinnamon, ginger, nutmeg, cloves, salt, coconut sugar, flax, and chia.
In another bowl, whisk together 2% milk, eggs, vanilla, and melted butter.
Pour wet ingredients into the dry and stir to combine.
Fold in the diced plums and nuts (if using). Spread mixture evenly into the prepared dish.
In a small bowl, combine flour, coconut sugar, and cinnamon. Cut in the cold butter with a fork or your fingers until coarse crumbs form.
Sprinkle crumble evenly over the oatmeal mixture.
Bake for 40–45 minutes, until golden brown on top and set in the center.
Cool slightly before slicing into squares. Dust with powdered sugar before serving, if desired.