Prep: Preheat the oven to 350°F. Lightly grease an 8×11 baking dish or a 9-inch square baking dish. Set aside.: Preheat the oven to 350°F. Lightly grease an 8×11 baking dish or a 9-inch square baking dish. Set aside. : Warmth fills the kitchen as the oven comes up to temperature, and the light scent of heat primes the butter and sugars you will brown later, so preheating matters. I notice the oven is ready when the dial or digital readout reaches 350°F , and the air feels evenly warm if you carefully touch the oven door for a moment. Greasing the dish prevents the filling from sticking and makes serving easier, so use a thin layer of butter or a neutral spray. A common mistake is skipping the greasing; when that happens the edges can cling and break apart when you serve. If your oven runs hot, check it a bit early to avoid overbrowning.
Topping: Add all of the topping ingredients to a large bowl EXCEPT the butter and stir until combined. Pour in the melted butter and stir with a spatula to combine then use your hands as needed to combine. Transfer to the refrigerator while you prep the filling.: Add all of the topping ingredients to a large bowl EXCEPT the butter and stir until combined. Pour in the melted butter and stir with a spatula to combine then use your hands as needed to combine. Transfer to the refrigerator while you prep the filling. : At this stage you want to notice the dry aroma of the oats and the sweet perfume of the light brown sugar , which signals they are well mixed. When the melted butter hits the dry mix it should moisten the particles but not make them soggy; think clumps and loose crumbs. Using your hands helps you form small clusters, which bake into those prized crunchy bits. Chilling the topping firms the clusters so they hold up better during assembly. A common slip is overmixing once the butter is added, which can produce a paste instead of crumbly clusters; stop as soon as you see small clumps.
Cut Strawberries: It is important that the strawberries aren’t cut too small otherwise they can practically disintegrate! You will want to leave super small strawberries whole, halve small berries and quarter large berries. We are aiming for about 3/4-1-inch-wide pieces. Transfer sliced strawberries to a large bowl.: It is important that the strawberries aren’t cut too small otherwise they can practically disintegrate! You will want to leave super small strawberries whole, halve small berries and quarter large berries. We are aiming for about 3/4-1-inch-wide pieces. Transfer sliced strawberries to a large bowl. : The texture of the finished filling depends on how you cut the strawberries . When you slice to roughly 3/4 to 1 inch, the fruit still offers pleasant tooth and visual appeal while releasing enough juice to meld with the thickener. You'll see bright red flesh exposed and smell fresh berry perfume when they're cut correctly. If pieces are too tiny they dissolve into a jammy mass, which is not wrong but different. A frequent error is overcutting; err on the larger side for a more pleasing bite.
Filling: Add the remaining Strawberry Filling ingredients to the strawberries and stir until evenly combined. Transfer filling to the prepared baking dish and spread into an even layer. Sprinkle the topping over the filling, breaking up any large chunks (you still want some small chunks).: Add the remaining Strawberry Filling ingredients to the strawberries and stir until evenly combined. Transfer filling to the prepared baking dish and spread into an even layer. Sprinkle the topping over the filling, breaking up any large chunks (you still want some small chunks). : Once you add granulated white sugar , cornstarch , lemon juice , and vanilla extract to the berries, the mixture should look glossy and slightly thickened as the cornstarch coats the fruit. Stir gently so juices are distributed and the coating is uniform. Spread it evenly in the dish to ensure consistent baking; uneven thickness causes some spots to bubble sooner than others. When you scatter the topping, aim for a mix of large and small clusters; visually you want golden pockets surrounded by red peeking through. A common mistake is pressing the topping down; keep it loose so air can circulate and the topping crisps.
Bake uncovered, at 350°F for 35-45 minutes, until the topping is golden and the strawberry juices are bubbling vigorously around the edges. Remove from the oven and let cool on a wire rack for 10 minutes before serving. Serve with vanilla and extra cinnamon if desired.: During baking, your kitchen will fill with the nutty aroma of toasted oats and the sweet perfume of cooked strawberries . Watch for the topping to develop a deep golden brown, and for the filling to bubble vigorously at the edges, which indicates the juices are thick and hot. The sound is a lively simmering and the visual cue is glossy fruit and bubbling syrup. Letting it rest for about ten minutes helps the filling set so slices hold their shape when scooped. One frequent misstep is pulling it from the oven too early; the topping may look pale and the filling undercooked. If the top browns too quickly, tent loosely with foil to finish cooking the interior.