Heat oven to 350 degrees F. Coat 2-1/2-quart baking dish with nonstick cooking spray.
In skillet, melt butter. Add crumbs, garlic and sage; cook until golden, 2 minutes. Remove from heat. Add 1/4 cup Parmesan and salt.
Cook pasta following package directions.
In a bowl, mix mascarpone, sour cream, 3/4 cup Parmesan, sage, salt, and pepper. Drain pasta; add to cheese mixture. Place in baking dish. Top with crumb topping.
Bake, uncovered, in 350 degree F oven 25 minutes. Let stand 10 minutes. Makes 8 servings.