Heat the oven to 200 C / 400 F. Set a 23 cm / 9-inch oven-safe baking dish on the counter.
Place the feta block in the center of the dish. Scatter the grapes around it.
Drizzle the olive oil and 2 tbsp of the honey over the feta and grapes. Sprinkle the thyme leaves, red pepper flakes, and black pepper on top.
Roast for 18 to 22 minutes, until the feta is soft to the touch and lightly golden on top, and the grapes have blistered and released their juice. Check at 18 minutes since ovens vary, the feta can dry out if left too long.
Remove the dish from the oven and let it sit for 2 minutes.
Drizzle the remaining 1 tbsp honey over the warm feta. Scatter the chopped walnuts and a few fresh thyme leaves on top.
Serve warm, straight from the dish, with crusty bread or crackers for scooping up the feta and grapes.