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 ½ 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.