Put an oven rack in the center of the oven. Preheat the oven to 350 degrees F. Butter an 8 by 8-inch glass baking dish. Set aside.
For the filling: In a medium bowl, whisk together the lemon juice and arrowroot until smooth. Add the strawberries, peaches and brown sugar. Gently toss until the fruit is coated. Pour the fruit mixture into the prepared pan.
For the topping: In the bowl of a food processor, combine the flour, oats, almonds, brown sugar, cinnamon, and sea salt. Pulse until mixed. Add the butter. Pulse until the butter is the size of peas. Sprinkle the mixture over the filling and bake for 40 to 45 minutes until the filling is bubbling and the top is light golden. Cool the crumble for 5 minutes.
Spoon the warm crumble into bowls and serve with a dollop of whipped cream or ice cream.
The crumble can also be made using 1 pound frozen and thawed whole strawberries and 1 pound frozen and thawed peach slices (plus any thawing juices). Reduce the arrowroot to 1 tablespoon and cook as above.