1. Combine milk, half-and-half, lemon verbena, and sugar in a medium, heavy saucepan over medium-high heat. Heat milk mixture to 180° or until tiny bubbles form around edge of pan (do not boil). Remove from heat; let stand 2 minutes. Strain through fine sieve over bowl; discard solids.
2. Place egg yolks in a large bowl; stir with a whisk. Gradually add half of hot milk mixture to yolks, stirring constantly with a whisk. Pour yolk mixture and remaining milk mixture into pan. Cook custard over medium heat 5 minutes or until thermometer registers 160°, stirring constantly. Remove from heat; cool to room temperature.
3. Pour custard into freezer can of an electric ice-cream freezer; freeze according to manufacturer's instructions.