Place the mascarpone in a small bowl, add the heavy cream a little at a time, and slowly whisk to combine. Add 1 tablespoon of the honey, the sumac, black pepper, cloves, and salt and gently whisk to combine. Set aside until ready to use.
Heat a grill (or grill pan) over high heat.
Place the plums and the oil in a small bowl and toss to combine. Place the plums onto the grill and cook until they have golden brown grill marks, 2 to 3 minutes. Turn the plums 90 degrees and grill for another 2 minutes.
Divide the plums among four plates and drizzle with the remaining ¼ cup of wildflower honey. Place a dollop of the mascarpone on each plum, sprinkle on the hazelnuts and lemon balm, and serve immediately.