To prepare the short ribs, preheat the oven to 350°F. Bring the short ribs to just above room temperature.

In a large Dutch oven, warm the vegetable oil. Season the short ribs with the salt and pepper. Dredge the ribs in flour, coating them lightly. Add to the pot and brown the ribs on both sides. Transfer to a plate. 

Add the onions, carrots, celery and garlic to the pot and cook over medium heat, stirring occasionally, for 3 minutes. Add the tomato paste and cook, stirring occasionally, for 3 minutes. While whisking, slowly add the 3 tablespoons flour until the sauce is smooth. Slowly add the wine, stock and bay leaves.  Bring the sauce to a boil, then return the ribs to the pot. Simmer, uncovered, for 20 minutes. 

Cover the pot, transfer to the oven and cook until the ribs are tender, 2 to 2 1/2 hours.

Transfer the ribs to a cutting board. Place the pot with the sauce on the stovetop over medium heat and simmer until reduced by about half. Chop the meat into small pieces, about 1 inch square, and return them to the reduced sauce.

To prepare the three-cheese mac, preheat the oven to 350°F. 

Bring a large pot of lightly salted water to a boil over high heat. Add the macaroni and cook according to the package instructions (usually 8 to 10 minutes). Drain and set aside.

In a medium saucepan, melt the butter until just melted (do not let it burn!), then slowly and thoroughly whisk in the flour to make a roux (the French term for a base sauce of butter, milk and flour, which may then be embellished upon). Slowly whisk in the milk until smooth. Cook on medium to low heat for about 10 minutes. 

Add the mustard powder and all of the cheeses to the pan and cook, stirring, until the mixture is well combined, about 3 minutes. Remove from the heat.

Transfer the macaroni to a casserole dish and fold in the peas, then pour the cheese sauce over the pasta. Taste and adjust the seasoning. Cover and bake for 25 minutes. Remove from the oven and keep covered until ready to serve.

To serve, spoon a portion of the mac and cheese into a bowl and top with the short ribs. Drizzle a little sauce over the ribs and pasta and serve warm.