Slice the baguette into ½ inch slices. You should get about 16 crostini pieces.
With a silicone brush, brush each side of the crostini pieces lightly with olive oil. Place on a sheet pan in one layer.
Place under the broiler and broil until browned on one side, about 2 - 3 minutes but watch them carefully. Flip and brown on the other side. The crostini should be lightly browned and crunchy.
Remove from the sheet pan and allow to cool. This can be done a day or two ahead. Once cooled, store in a sealed container until you are ready to use them.
Roasting the Grapes
Preheat the oven to 425°F.
Wash the grapes well and remove any stems. Dry them with a paper towel if necessary.
Place the grapes on a single layer in a roasting pan or sheet pan. Toss with olive oil or grapeseed oil to coat them thoroughly.
Sprinkle on the rosemary and sea salt and toss well.
Place them in the middle of the preheated oven and roast them for 15 - 25 minutes, until they are softened and break apart with just a little pressure applied by the back of a fork.
Remove from the oven and allow them to cool.
Assemble the crostini
Spread each crostini piece with ~1-½ tsp. of mascarpone cheese.
Add 3 - 5 roasted grapes on top of the cheese, depending on the size of the grape and the crostini pieces.