1. In a shallow 2-quart baking dish, combine 1 1/4 cups sugar and tapioca. Add rhubarb, raspberries, and lemon juice - mixing gently but thoroughly. Let stand from 15 minutes to one hour to soften tapioca, mixing several times.
2. 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 to create fine crumbs.
3. Squeeze matzo mixture to compact into lumps, then crumble over rhubarb mixture.
4. Baked in a 375°F oven until cobbler is bubbling in center and top is golden brown, about 1 hour. Let cool 15 minutes.
5. Spoon cobbler into bowls and top with sorbet or whipped cream (optional).