Preheat the oven to 375 degrees F. Lightly butter the the ramekins (or spray with cooking spray) and place them on a baking sheet.
Make the crumble topping: Combine the flour, brown sugar, and salt in a bowl. Rub the butter into the flour mixture using your fingers, until the mixture is crumbly, with pea-sized bits of butter. (You can use a mini-prep if you have one, or a fork and a knife, but I think it’s easier just to use your fingers!) Toss the walnuts into the mixture until combined. Set aside.
In a separate bowl, toss the strawberries, rhubarb, sugar, orange zest and juice in a bowl. Divide the strawberry mixture between the ramekins and sprinkle with the crumble topping.
Bake the crumbles for 25 minutes or until the tops are golden brown and the fruit mixture is bubbling. Let cool slightly and serve with vanilla ice cream.