First, make the cherry filling. To do this, combine cherries, sugar, and corn starch in a 9x13 baking pan. Add the ingredients in and toss to coat evenly.
Next, make the crisp topping. Either use room temperature butter or melt it slightly in the microwave. In a medium bowl add the softened butter, oats, flour, sugars, salt and cinnamon. Using your hands or a fork, combine everything together until it is crumbly and the butter is evenly spread throughout the mixture.
Add the crisp topping on top of the cherry mixture and evenly coat.
Bake in the preheated oven for 45-50 minutes or until the top is golden brown.
Let it cool for about 15 minutes and then spoon into a bowl and top with vanilla ice cream if you'd like and enjoy! The filling will thicken up the more it cools.
Notes
Store the crisp in an airtight container or cover with plastic wrap. Keep the crisp in the fridge for about one week. To eat again, you can heat in the microwave for about 30 seconds or until warm. If the oats have lost their crispness, you can also add it to an oven-safe container and bake at 350F until warmed through and the top crust is crispy, about 10 minutes.
This topping for the crisp is thicker, if you prefer less topping you can half the topping ingredients.
If you do not have fresh tart cherries, you can use canned tart or sour cherries instead, just be sure cherries are the only ingredient.
The crisp is delicious on it's own, or topped with a scoop of vanilla ice cream.