Heat oven to 475°F. On a rimmed baking sheet, toss grapes with lemon juice, 2 tablespoons syrup, and 1 tablespoon oil, then thyme sprigs, and roast 8 minutes.
Drizzle with 2 tablespoons water, shake pan, and roast until grapes begin to darken and shrivel slightly, 6 to 8 minutes.
While grapes are roasting, place a 10-inch cast-iron skillet on the other rack in oven to heat (heat only 8 minutes).
While skillet is heating, prepare batter: In blender, blend Just Egg, cashew milk, vanilla, and remaining ¼ cup maple syrup.
In a small bowl, whisk together flour, baking powder, and salt. Add to blender along with ⅓ cup whole hazelnuts and blend to finely grind hazelnuts, 10 to 20 seconds.
Reduce oven temp to 425°F. Working quickly, add remaining 3 tablespoons oil to hot skillet, swirling to coat, then pour in batter. Return to oven and bake until golden brown and just set (should not be super firm), 25 to 30 minutes. Serve immediately, topped with grapes (drizzling with any pan juices), chopped hazelnuts, and thyme leaves.
