Preparation

• Position a rack in the center of the oven and heat the oven to 375ºF.

Make the topping

• In a large saucepan, boil the coconut milk over medium heat, stirring occasionally, until reduced to 3/4 cup, about 15 minutes. Set aside to cool completely.
• In a large bowl, whisk the flour, sugar, baking powder, and salt until well blended.
• In a medium bowl, whisk the reduced coconut milk, egg, and yolk until well blended. Add the coconut mixture and the melted butter to the flour mixture; gently fold together with a spatula. Refrigerate.

Make the filling

• In a large bowl, toss the pluots, sugar, cornstarch, and salt with a spatula.
• In a 10-inch nonreactive, ovenproof skillet (8- to 10-cup capacity), melt the butter over medium-low heat. Add the pluot mixture, scraping the bowl clean. Cook, stirring, until the fruit begins to give off some juice and the liquid is boiling, about 3 minutes. Add the blueberries and rum ( if using). Gently toss until the berries are hot, about 1 minute. Remove from the heat.

Assemble and bake

• Spread the fruit into a relatively even layer. Using a 2-tablespoon scoop (or two tablespoons), drop the topping randomly onto the filling. The topping will almost cover the fruit. Bake until the filling is bubbling and the topping is golden-brown, 40 to 50 minutes. Sprinkle with the toasted coconut and let sit for at least 30 minutes to allow the filling to settle and thicken before serving.