In a shallow 2 quart baking dish, mix 1 1/4 cups sugar and tapioca. Add rhubarb, raspberries, and lemon juice; mix gently but thoroughly. Let stand 15 minutes to 1 hour to soften tapioca; mix several times.
Meanwhile, in a food processor or a bowl, whirl or rub together with your fingers the matzo meal, the remaining 1/2 cup sugar, margarine, and nutmeg until fine crumbs form. Squeeze to compact into lumps, then crumble over rhubarb mixture.
Bake in a 375° F. oven until cobbler is bubbling in center and top is golden brown, about 1 hour. Let cool 15 minutes; spoon cobbler into bowls and top with sorbet, if desired.