Preheat oven to 450ºF. Toss grapes with 2 tbsp of olive oil and thyme sprigs then spread in an even layer on a baking sheet, leaving some stems attached if you like (it makes a pretty presentation). Roast for about 25 minutes, flipping grapes once, until caramelized and juicy. Let cool.
Brush baguette slices with more olive oil and arrange on a second baking sheet. Bake about 6 minutes, flipping once half-way through, until toasted on both sides.
Switch the oven to broil. Top each toast with a thick wedge of camembert and broil about 5 minutes, watching very, very closely, until cheese is golden at edges and just softened. Top each crostini with a small bunch of grapes and sprinkle with a few thyme leaves, if desired.