1. Preheat the oven to 350°F (175°C). Grease a 10-inch oven-safe skillet.
2. In a large bowl, combine the rhubarb, pineapple, granulated sugar, grated ginger, cornstarch, and vanilla extract. Mix well and spread evenly in the prepared skillet.
3. In another bowl, combine the rolled oats, flour, brown sugar, ground cinnamon, and salt. Add the melted butter and stir until the mixture is crumbly.
4. Sprinkle the oat mixture evenly over the fruit in the skillet.
5. Bake for 35-40 minutes, or until the topping is golden brown and the fruit is bubbling.
6. Let cool slightly before serving. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream, if desired.