1.Preheat oven to 350°F.
2. Place the berries into a large bowl and add the lemon juice, lemon zest, sugar and cornstarch. Mix all ingredients then pour into a 9” x 12” baking dish.
3. Prepare the topping: Combine the flour, sugars, salt, oatmeal and butter in the bowl of an electric mixer with the paddle attachment. Mix on low speed for about 1 minute until the mixture is in large crumbles and butter is the size of peas. If the mixture is not coming together, add cold water, 1 teaspoon at a time. This can also be done by hand using a pastry cutter. Sprinkle evenly on top of fruit, covering completely.
4. Place dish on a sheet tray to catch any juices that may spill over. Bake for 45 – 50 minutes until top is brown and fruit is bubbly.
5. Prepare the whipped cream: In a freezing cold metal bowl, whip the cream, lemon zest, lemon juice, vanilla, and sugar until soft peaks form, but do not over-whip. Use immediately or refrigerate, rewhipping may be necessary. An electric mixer is fine too.