Wash and cut the stems off the strawberries then cut the strawberries in halves or quarters. Once all the stems are cut off the strawberries should weigh around 570g.
Add the strawberries to a bowl then add the sugar and mix. Cover the bowl and set aside for around 1 – 2 hours. This process will draw out some of the water from the strawberries so you don’t have too much runny liquid once the crumble bakes. The longer it sits, the more water will be released from the strawberries.
Preheat the oven to gas mark 4/350oF/180oC/160oC fan. Strain the liquid from the strawberries into a small saucepan and add the strawberries to a dry bowl and set aside. To the saucepan, add the lemon juice and mix in the cornflour then place it on a low heat for 1 -2 minutes. Once the juice thickens, add it to the bowl with the strawberries and mix well. Pour the strawberries with the thickened sauce into an 8 x 8-inch baking dish then evenly spread and set aside.
Add the flour to a bowl along with the butter. Use your fingers and rub the butter into the flour until you have peas size pieces of butter. You can use a pastry cutter or food processor if you’d prefer not to use your hands. You can even do this with a fork. To the flour and butter mixture, add the oats, soft brown sugar, granulated sugar and salt and then combine. Sprinkle the topping onto the strawberries and evenly spread. The strawberries will still release more juice so ensure you leave a gap around the edge of the baking dish so some of the liquid can evaporate.
Bake in the middle of the oven for 50 minutes or until the topping is golden in colour and the strawberry juice is bubbling. Remove from the oven and allow to cool for 10 - 15 minutes then serve warm with whipped cream or ice-cream. You can reheat in the microwave if not serving immediately.