Heat the oven to 350 degrees.
Slice the baguettes in 1/4 inch rounds, brush lightly with olive oil, and place on a baking sheet. Toast the bread in the oven for 15-20 minutes, or until golden. Turn pan during baking or flip slices over if needed. Allow to cool.
Increase oven temperature to 425 degrees.
Line a rimmed baking sheet with parchment or a Silpat. Remove the grapes from the stem, and toss them together with the rosemary sprigs, pecans and 2-3 tablespoons olive oil over the baking sheet. Spread the grapes and nuts out so that they’re in a single layer, and place the baking sheet in the oven for 10-12 minutes, or until the grapes have just begun to wilt and a few have started to burst. Remove from the oven and season evenly with salt and pepper.
Spoon the roasted grapes and nuts onto the crostini, remove the rosemary from the stem, and add a bit of it to the top of each piece. Serve.