Start with the panna cotta by picking 8 1/3-cup glass cups, containers or silicone molds. Set aside.
In a small bowl, place lemon juice and sprinkle the gelatin powder over evenly. Let stand for 15 minutes until soften.
In a medium saucepan over medium-low heat, bring the heavy cream, sugar, lemon zest to a simmer (not a boil). Whisk in the gelatin until dissolved. Strain the mixture into a large glass measuring cup or a medium bowl. Divide the mixture among the prepared containers. Chill the panna cotta uncovered in the fridge until set, at least 4 hours. Cover with plastic wrap once cooled.
For the coulis, in a medium saucepan over medium heat, combine the grapes, lemon zest, lemon juice and a pinch of salt. Cook for about 45 minutes or until the grapes are very soft and broken apart, stirring occasionally. Strain through a fine mesh sieve to a medium bowl. Discard the skins and flesh. Return the juice back to the saucepan. Cook the coulis over medium heat for another 5 minutes. Add honey and cook until slightly thicken, about 3 more minutes. Cool and transfer to an airtight container. Keep in the fridge until ready to use.
For the roasted grapes, preheat the oven to 375˚F (190˚C). In a medium baking pan, add the grapes and bake for 25 - 30 minutes. Remove from heat. Cool and transfer to an airtight container. Keep in the fridge until ready to use.
To assemble, remove the panna cotta from mold if needed. Top the panna cotta with about 2 teaspoons of grape coulis and a few roasted grapes. Serve.