01 - Lightly oil or spray 6 small ramekins or glasses with 4–6 oz capacity for easy unmolding later.
02 - Sprinkle gelatin powder over 2 tablespoons cold water in a small bowl. Let stand for 5 minutes until absorbed and softened.
03 - Combine heavy cream, whole milk, granulated sugar, lemon zest, and salt in a saucepan. Heat over medium heat until just steaming, stirring constantly to dissolve sugar completely. Do not let the mixture boil.
04 - Remove saucepan from heat. Whisk in the bloomed gelatin until fully dissolved into the warm cream mixture.
05 - Stir in fresh lemon juice and vanilla extract. Whisk thoroughly until the mixture is completely smooth and homogeneous.
06 - Pour the mixture through a fine-mesh sieve into a measuring jug to remove any lumps or lemon zest pieces. Distribute evenly among the prepared ramekins.
07 - Allow panna cottas to cool to room temperature, then refrigerate for at least 4 hours or overnight until completely set and firm.
08 - Preheat oven to 400°F for roasting the grapes.
09 - Toss 1 cup of grapes with olive oil, honey, and thyme leaves if using. Spread on a parchment-lined baking sheet and roast for 10–12 minutes until softened, juicy, and caramelized. Let cool completely.
10 - Cut the remaining 1 cup of grapes in half. Keep chilled until ready to serve.
11 - Unmold panna cottas onto individual plates or serve directly in glasses. Arrange roasted grapes and their pan juices on one side, scatter fresh grape halves on the other, and garnish with additional lemon zest or thyme if desired.