Heat the oven to 350 F (180 C/Gas 4). Butter a 2 1/2-quart baking dish.
Using a large saucepan or stockpot, cook the noodles following package directions; drain well. Put noodles back in the pot or in a large bowl and set aside.
In a medium saucepan, melt 2 tablespoons of butter over medium heat; cook the chopped onion for 2 minutes.
Add the corned beef and cook until lightly browned.
With a slotted spoon, transfer beef and onion mixture to the pot with noodles. Add the squeezed spinach to the pot and set aside.
In the same medium saucepan, melt the remaining 2 tablespoons butter over medium-low heat; stir in the 3 tablespoons of flour and the mustard powder until well blended and bubbly. Continue cooking, while stirring, for 2 minutes.
Gradually stir in chicken broth and milk. Cook, stirring constantly, until thickened and bubbly. Add pepper and thyme. Stir in the cheese, cooking until melted. Add salt to taste.
Pour the sauce over the noodle and corned beef mixture and gently stir until well blended.
Spoon the corned beef and noodle mixture into the prepared baking dish.
If desired, top with crumbled French fried onions or buttered bread crumbs.
Bake for 25 to 30 minutes, or until hot and bubbly and the topping is lightly browned.