Preheat the oven to 350*F. Bring a large pan of lightly salted water to a boil and cook the macaroni until al dente(about 8 minutes). Rinse under cold water and drain.
Melt 2 tbls. of the butter in a pan and stir in the flour. Cook for 1-2 minutes, stirring. Gradually add the milk, whisking constantly until the sauce boils and thickens.
Stir in the mustard, cinnamon and 1 cup of the cheese. Cook gently, stirring often, until the cheese has melted, then remove from the heat and whisk in the egg. Set aside.
Melt the remaining butter in a small fryiing pan and cook the scallions, chopped tomatoes and corn over a gentle heat for 5-10 minutes. Add peas now if using. Season to taste.
Tip half of the cooked macaroni into a greased ovenproof dish. Pour over the other half of the cheese sauce and mix well. Then spoon the tomato and corn mix over the macaroni.
Stir the remaining macaroni into the remaining cheese sauce, then spread this carefully over the tomato and corn mixture. Top with the rest of the cheese. Bake for 45 minutes, or until the top is golden and bubbling.