Preheat the oven to 350 degrees F. Place the sliced baguette rounds on a baking sheet. Bake the bread for 10 minutes, then flip the toasts over and bake for 5 minutes more. Remove the sheet from the oven.

While the bread is toasting, combine the mascarpone, pumpkin, salt, pepper, and nutmeg in a bowl and stir together until smooth.

Add the hazelnuts to a small skillet and heat them over medium-low heat. Toast the hazelnuts until golden and fragrant, 3 to 5 minutes, occasionally shaking the pan or stirring the nuts so they don't burn. Remove the pan from the heat immediately and place the nuts in a bowl. Once the hazelnuts have cooled, coarsely chop them.

Wipe out the same skillet with a paper towel and heat it over medium-low heat. Add the butter and, once it's melted, add the sage leaves. Cook the sage leaves until golden and crispy, flipping after 30 seconds. Transfer the leaves to a paper towel to cool slightly.

To assemble the crostini, spread the pumpkin-mascarpone mixture over the toasts. Cover each toast with a sprinkling of the chopped hazelnuts. Crumble the fried sage between your fingers and sprinkle it over the toasts.