Kefir Ice Cream Base: Add lemon juice, zest, sugar, and thyme to a small saucepan and set over medium heat. Cook until sugar dissolves, 3 to 5 minutes. Remove from heat and let cool to lukewarm, then stir in kefir.
Churn: If using a self-cooling ice cream churner, pour mixture directly in and churn for 30 to 45 minutes, or until it resembles soft serve. If using a churner that requires you to chill it in advance, be sure to chill the kefir mixture in the fridge for at least 4 hours, or in the freezer for an hour before churning.
Freeze: Transfer to a freezer-safe container and flatten with the back of a spoon. Cover with plastic wrap and freeze until firm, at least 4 hours. Set at room temperature for a few minutes before serving.