1. Carefully remove rind from 2 oranges using a vegetable peeler; discard white pith. Cut rind into 1 x ¼-inch-thick strips. Cut peeled oranges in half; use a citrus reamer to squeeze juice from orange halves. Repeat with remaining oranges until juice measures 2 2/3 cups.
2. Combine 2 1/2 cups water and sugar in a small saucepan; bring to a boil. Add rind strips to pan. Reduce heat; simmer for 5 minutes. Strain sugar mixture through a sieve over a bowl, reserving liquid; discard solids. Cool completely.
2. Add orange juice and lemon juice to sugar mixture; stir well. Pour mixture into the freezer can of a tabletop ice-cream freezer; freeze according to manufacturer's instructions. Spoon sorbet into a freezer-safe container; cover and freeze for 1 hour or until firm. Garnish with grated rind and mint sprigs, if desired.