On a rimmed baking sheet add grapes, thyme, olive oil, kosher salt, and pepper. Mix together until the grapes are coated.
Roast in the oven for about 20 minutes or until the grapes have started to shrivel and have released some of their juice.
Let the grapes cool slightly while you prepare the crostini.
Lay out toasted bread and spread each piece with the desired amount of ricotta. I prefer around 1 tablespoons per slice. Top each slice with a spoonful of roasted grapes, a crack of fresh pepper, and a few fresh thyme leaves if desired.