Add Post Shredded Wheat to a food processor. Pulse on/off until cereal resembles coarse breadcrumbs.
Preheat the oven to 350ºF.
Bring a large pot of water to a boil. Add the pasta and cook until al dente. Drain and set aside.
Tie the bunch of thyme and sprigs of rosemary together with clean kitchen string. In a large widemouth stockpot, melt the butter over medium heat.
Add the onions, garlic and herbs, and cook until the onion is soft, 8 to 10 minutes. Add the wine and cook, stirring occasionally, until reduced by half, about 10 minutes. Remove and discard the garlic and herbs.
Whisk in the flour and cook, stirring, for 1 minute until liquid is smooth, no lumps. Gradually whisk in the milk and cream, then the mustard. Reduce the heat to a gentle simmer and cook, stirring constantly, for about 30 minutes.
Remove the pan from the heat and stir in the cheddar, a handful at a time, waiting until each addition is completely melted before adding more. Season with the nutmeg and add salt and black pepper to taste.
Add the pasta, and the chopped thyme and heat through until the pasta is well coated and seasoned, about 5 minutes. Adjust the seasonings as needed.
Pour the mixture into a casserole dish or individual ovenproof gratin dishes. Sprinkle the top(s) with a generous layer of coarsely ground Post Shredded Wheat then a layer of Grana Padano, then another layer of Post Shredded Wheat.
Bake for 20 minutes, or until bubbly and crispy.