Heat oven to 350 degrees F. Lightly coat the inside of a shallow 2 1/2-quart baking dish with nonstick vegetable-oil cooking spray.
Cut bread into small cubes. (Or, tear bread into large pieces; place in food processor and pulse until coarse crumbs.) Spread bread in even layer on a jelly-roll pan. Bake in 350° oven for 10 minutes or until the cubes or crumbs are golden brown. Remove from oven; leave oven at 350 degrees F.
Meanwhile, bring a large pot of water to a boil. Add the sliced onion and pasta; cook for 8 to 10 minutes or until pasta is firm, but tender.
While the pasta is cooking, in a large bowl, whisk together the mascarpone, herb cheese, milk, salt, pepper, nutmeg and cayenne until well blended. Stir in the shredded cheddar until well mixed.
Drain the pasta and onion in a colander. Add to the cheese mixture in the bowl; toss to coat the pasta evenly and melt the cheddar. Transfer the pasta mixture to the prepared baking dish. Evenly sprinkle the toasted crumbs and Parmesan cheese over the top.
Bake in 350 degree oven for 30 minutes or until the filling is bubbly. Remove the baking dish to a wire rack; let cool 10 minutes. Serve warm.