Preheat your oven to 350°F (175°C). Wash and trim fresh rhubarb, then chop it into half-inch pieces.

In a large mixing bowl, toss the rhubarb with granulated sugar, cornstarch, vanilla extract, a pinch of salt, and optional lemon juice. Let the mixture sit for about 10 minutes.

In another bowl, combine old-fashioned oats, all-purpose flour, brown sugar, ground cinnamon, and a pinch of salt. Drizzle in the melted unsalted butter and mix until crumbly.

Spread the rhubarb filling in an 8x8 inch baking dish and sprinkle the oat topping over it.

Bake for 45 minutes, or until bubbly and golden brown. Remove and let cool for 15 minutes before serving.