Preheat oven to 350°F.: As the oven warms, notice the quiet hum and the steady climb of temperature that will coax the juices from strawberries and brown the topping. Preheating ensures even baking and correct timing, so do not skip this. A common slip is starting with a cold oven which can lead to soggy topping and uneven bubbling, so wait until the oven reaches the full temperature.
Grease a 9 1/2-inch deep dish pie plate or a similar-sized baking dish. Set aside.: The act of greasing the dish prevents sticking and helps the browned edges release cleanly. Rub a little softened unsalted butter or use a neutral bake spray, focusing on corners for easy serving later. If you skip greasing, the fruit juices can glue to the pan and make serving difficult, so be thorough.
In a large bowl, combine all of the filling ingredients and stir gently. Dump the mixture into the prepared baking dish and distribute it evenly.: You will see strawberries shimmer as the granulated sugar and lemon juice mingle, and the vanilla extract scents the mixture. Stirring gently prevents bruising the berries while ensuring the small amount of all purpose flour disperses evenly to thicken the filling as it bakes. A common mistake is overmixing which can pulverize berries, resulting in a too thin sauce, so be gentle and even when spreading in the dish.
In a large bowl, combine oats, flour, both sugars, salt, cinnamon, and lemon zest. Pour melted butter on top and stir with a fork until well combined.: The dry ingredients should look sandy before the butter arrives; once the melted unsalted butter hits the bowl, steam and a buttery aroma will rise and the mixture will clump into small clusters. These clumps are what bake into crisp, golden pockets. Stirring until the oats are coated ensures even browning. If you overwork the mixture you can dissolve the sugar too much or make it soggy, so stop as soon as clusters form.
Sprinkle the topping over the strawberry mixture.: As you scatter the oat topping, notice how it sits atop the berries, creating islands that will brown at different rates, producing variation in texture. Drop larger clumps for crunch and fine crumbs for a delicate finish. Pressing the topping down will create a denser crust, while leaving it loose yields a crumbly texture. A mistake is piling too thin a layer which can burn quickly, so aim for even coverage.
Bake for 30-40 minutes or until the fruit is bubbling and juicy and the topping is light golden brown.: During baking you will hear tiny pops as juices bubble and see steam escaping, and the topping will change color from pale to golden, smelling nutty and caramelized. These visual and olfactory cues tell you the dish is ready. Avoid underbaking which leaves the oats raw, or overbaking which can dry out the fruit and toughen the topping. If the top is browning too fast, tent with foil for the final minutes to protect it while the filling finishes bubbling.
Cool slightly, and serve warm with vanilla ice cream.: Letting the crisp rest for a short time allows the filling to set so you can spoon tidy portions, while still warm enough to melt a scoop of Vanilla ice cream . The aroma will be rich with toasted oats, cinnamon, and lemon, and the contrast between hot fruit and cold ice cream is part of the magic. Serving it piping hot can cause the ice cream to melt too quickly, so wait a modest five to ten minutes for best texture.