Preheat the oven to 375ºF. Prepare a 9x9" pan.
In a large bowl, mix together the filling: blueberries, lemon zest, lemon juice, sugar, thyme, cornstarch, and a pinch of salt. Mix until incorporated then transfer to the 9x9" baking pan.
In a separate bowl, mix together the flour, thyme, rolled oats, brown sugar, lemon zest, and a pinch of salt. Add in the butter pieces and use your hands or a pastry cutter to mix altogether until you get a sand-like mixture.
Scatter the topping over the blueberry mixture.
Bake for 35-40 minutes until the topping is golden brown.
Let the crumble cool for 10-15 minutes. Serve with whipped cream or vanilla ice cream.