Preheat oven to 350 degrees. Spread the hazelnuts on a baking sheet and toast them in the oven, jostling the pan every few minutes, until they are lightly browned and giving off a nutty aroma, about 8 to 10 minutes. Remove and transfer the nuts to a clean kitchen towel. Enclose them in the towel and rub as hard as you can to loosen the skins (though it’s okay if some hazelnut skins remain on). Discard the skins. When the nuts have cooled slightly, coarsely chop them and set aside.
Preheat the broiler.
In a medium bowl, use a heavy spoon to mix together the mascarpone and gorgonzola until just combined (do not overmix). Gently stir in the chopped hazelnuts.
Arrange the bread slices on a baking sheet and broil a few minutes on each side until golden brown. Remove from oven. Brush slices lightly with olive oil and spread about 1 tablespoon of gorgonzola mixture on each slice. Sprinkle each with just a few flakes of coarse salt. Serve immediately.