Carefully remove rind from 2 oranges using a vegetable peeler; discard white pith.
Cut rind into 1 x -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.
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.
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.
