01 - Lightly oil or spray 6 small ramekins or glasses (4-6 oz capacity).
02 - Sprinkle gelatin over 2 tbsp cold water in a small bowl. Let bloom for 5 minutes.
03 - Combine cream, milk, sugar, lemon zest, and salt in a saucepan. Heat over medium until just steaming, stirring to dissolve sugar. Do not boil.
04 - Remove from heat. Stir in bloomed gelatin until completely dissolved.
05 - Add lemon juice and vanilla; whisk until smooth.
06 - Strain through a fine mesh sieve into a jug.
07 - Pour evenly into prepared ramekins. Cool to room temperature, then chill at least 4 hours or overnight until set.
08 - Preheat oven to 400°F.
09 - Toss 1 cup grapes with olive oil, honey, and thyme. Arrange on parchment-lined baking sheet. Roast 10-12 minutes until soft, juicy, and caramelized. Cool completely.
10 - Halve remaining 1 cup grapes and set aside chilled.
11 - Unmold panna cottas onto plates or serve in glasses. Spoon roasted grapes and juices on one side; scatter fresh grape halves on the other. Garnish with lemon zest or thyme if desired.