Twelve ounces of spaghetti, a cheddar cream sauce built in the same pan as the mushrooms, and two cups of diced leftover ham. Everything happens in one skillet, and the dish goes under the broiler for two minutes at the end for the golden top. Ham tetrazzini for six, on the table in about forty-five minutes.
Boil the spaghetti: Bring a large pot of salted water to a boil and cook the spaghetti until just under al dente. Reserve 1/2 cup of the pasta water, then drain and set aside.
Cook the mushrooms and onion: In a large skillet over medium heat, melt 2 tablespoons of the butter. Add the onion and mushrooms and cook 5 to 6 minutes, until softened and browning at the edges. Stir in the garlic and cook 1 minute more, just until fragrant.
Make the roux: Add the remaining 1 tablespoon butter to the pan. Once it melts, sprinkle in the flour and stir constantly for 1 to 2 minutes, until a golden paste forms.
Finish the sauce: Whisk in the milk gradually, then the chicken broth, keeping the mixture smooth. Stir in the thyme and simmer 3 to 4 minutes, stirring, until the sauce thickens but still pours.
Fold In the Cheese, Ham and Peas
Melt the cheddar and add the filling: Turn the heat to low. Stir in the shredded cheddar until fully melted and creamy, then fold in the diced ham and thawed peas. Season with salt and black pepper, tasting as you go, and remember the ham is already salted.
Toss the spaghetti through: Add the drained spaghetti to the pan and turn it gently until every strand is coated. If the mixture is too thick, loosen it with a splash of the reserved pasta water.
Broil and Serve
Brown the top: Transfer to a greased 9x13-inch baking dish and sprinkle the Parmesan evenly over the surface. Broil 2 to 3 minutes, until bubbling, golden and crisp at the edges. Stay in front of the oven, because 2 minutes browns it and 4 minutes burns it.
Rest and serve: Let the dish rest 5 minutes so the squares hold their shape, then sprinkle with chopped parsley and serve warm.
Notes
Cook the spaghetti to just under al dente. It keeps cooking in the hot sauce, and a minute of overcooking at the boil becomes three minutes of mush in the dish.Grate the cheddar from a block and stir it in over low heat. Pre-shredded cheese is coated with starch that turns a smooth sauce grainy.Use low-sodium broth and season at the end. Cured ham carries a lot of salt on its own.To make it ahead, assemble everything, stop before the broiler, cover and refrigerate up to 1 day. Take it out 15 minutes before baking, bake covered at 350 degrees for about 25 minutes, then uncover, add the Parmesan and broil.Leftovers keep 3 to 4 days in the fridge. Reheat with a tablespoon of milk stirred through. To freeze, assemble without broiling and freeze up to 2 months, then thaw overnight and bake covered for 40 minutes.Swap the ham for turkey or chicken cup for cup, or the peas for 2 cups of small broccoli florets.Nutrition values are an estimate.