In a small bowl, stir together the strawberries, sugar, cornstarch, and vanilla until the strawberries are evenly coated and no dry cornstarch remains.
In a second bowl, stir together the flour, oats, and brown sugar. Add the softened butter and work it in with a fork or your fingertips until the mixture forms small, crumbly clumps.
Spoon the strawberry filling into a 10-ounce ramekin and scatter the topping evenly over the fruit. Set the ramekin on a small rimmed baking sheet to catch any drips.
Bake for 30 to 35 minutes, until the topping is deep golden and the filling is bubbling around the edges. Let cool for a few minutes before serving so the filling can set.
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Notes
Drain frozen strawberries well. If you're using frozen strawberries, thaw them completely and drain off the liquid before mixing the filling. Skipping this step leaves too much water in the ramekin and the filling turns out thin instead of thick.Use softened butter, not melted. Softened butter blends into the flour and oats in small pieces, which is what gives the topping its crumbly texture. Melted butter makes the topping bake into a flat, solid crust.Set the ramekin on a baking sheet. The filling bubbles up as it bakes and can spill over the rim. A small rimmed baking sheet underneath catches any drips and keeps your oven clean.Cover with foil if the top browns too fast. If the topping reaches deep golden before the filling is bubbling, lay a piece of foil loosely over the ramekin and keep baking until the filling is done.Let it cool a few minutes before eating. The filling thickens as it cools and the cornstarch sets. Straight out of the oven it's loose and very hot, so a short rest gives you a better texture and a safer first bite.If doubling the recipe, double all the ingredients and bake in two 10-ounce ramekins, one 5×5-inch baking dish, one 4×6-inch baking dish, or one 6.5-inch round skillet.