Put oven rack in middle position and preheat over to 425° F.
Stir together sugar and potato starch, then gently toss with strawberries, rhubarb, lemon juice, and zest.
Transfer mixture to an ungreased shallow 3-quart (13- by 9- by 2-inch) baking dish.
Whisk together matzo cake meal, brown sugar, potato starch, cinnamon,and salt in a bowl.
Blend in butter with your fingertips until mixture forms small clumps.
Crumble streusel evenly over top of compote and bake until fruit is bubbling and topping is golden, 40- 45 minutes.
Cool to warm on a rack before serving.