Arrange a rack in the middle of the oven and heat the oven to 375°F. Coat an 8 or 9-inch square baking pan (or 2- to 2 1/2-quart baking dish) with some melted butter; set aside.

Make the filling:

Whisk 1/2 cup granulated sugar and 2 tablespoons cornstarch together in a large bowl until lump-free. Add 2 pounds fresh rhubarb pieces and 1 tablespoon freshly squeezed lemon juice and toss gently to coat. Transfer to the prepared baking dish.

Make the topping:

Wipe out the bowl. Add 1 1/2 cups old-fashioned rolled oats, 1/2 cup all-purpose flour, 1/2 cup packed light brown sugar, 1/2 teaspoon kosher salt, and 1/2 teaspoon ground cinnamon, if using, and stir until combined. Drizzle 12 tablespoons (1 1/2 sticks) melted unsalted butter over the oat mixture and stir until evenly moistened. Scatter the crisp topping evenly over the fruit mixture, leaving large clumps intact.

Bake until the fruit juices are bubbling around the edges of the baking dish and the topping is golden and firm to the touch, 35 to 45 minutes. Let the crisp cool on a wire rack for at least 15 minutes before serving. Serve with vanilla ice cream or freshly whipped cream, if desired.