Preheat the oven to 375°F with a rack in the middle. In a large bowl, toss together the cherries, sugar, cornstarch, vanilla extract and lemon juice until well combined. Set eight wide-mouth half-pint canning jars on a heavy, rimmed baking sheet and divide the filling equally among them.
In a medium bowl, combine the oats, flour, muscovado or brown sugar, and salt. Stir together thoroughly. Add the diced butter and, using your fingers, pinch the ingredients together until they form a cohesive, crumbly topping and no lumps of butter or loose bits of flour or sugar are left. Divide the topping evenly among the jars.
Bake for about 40 minutes, until the topping is browned and the filling is bubbly. Alternatively, you can freeze lidded, unbaked jars for up to 6 months and bake individually in a toaster oven when you're craving a little treat. You can bake directly from the freezer, but you may need to add a few minutes to the baking time.