Preheat your oven to 375 degrees F. Lightly spray an 8x8-inch square baking dish or individual ramekins (4 to 6 of them, depending on how big of a serving you want in each) with cooking spray. Set aside.
In a large bowl, combine the chopped rhubarb, sugar, and cornstarch, and stir to combine.
In a second medium bowl, combine the sugars, flour, oats, and pecans. Stir with a fork to combine.
Add the vanilla and melted butter, and stir until no dry powder is left.
Stir the rhubarb and sugar again and then place it in your baking dish, or divide it evenly among the ramekins. Place the crisp topping on top of the rhubarb in the dish. If you used ramekins, place them on a baking sheet and then transfer them to the hot oven, or put the baking dish straight into the oven. Cook for 18 to 24 minutes, until the crisp is lightly browned and the rhubarb is bubbling around the edges.
Let the crisp cool for 5 minutes before serving hot with ice cream or fresh whipped cream.
Notes
This crisp is best enjoyed the same day. Reheat it in a 350 F oven for 10 minutes.
You can use red or green rhubarb for this recipe.
Serve it with a scoop of vanilla ice cream or dollop of freshly whipped cream.
To store, let it cool completely then cover and store in the refrigerator.