Preheat oven to 375℉. Place an oven rack in the top third of oven and another rack in center of oven. Line a sheet pan with parchment paper.
In a medium bowl stir together oats, flour, brown sugar, cinnamon, and salt. Add butter and use your fingers to rub butter into flour mixture until evenly combined. Scatter crumble onto prepared sheet pan.
In a 9” square or round baking dish, toss plums with honey and thyme, then spread out evenly.
Place crumble on center oven rack and plums on top rack. Bake until crumble is browned and plums are caramelized, about 15 minutes for crumble and 30 minutes for plums. Allow crumble to cool completely.
To serve, spread 2 tablespoons yogurt on each Wasa, then top with plums, drizzling some of the juices over. Sprinkle with crumble.