Melt 2 Tablespoons of butter in a large pot over medium heat.
Once hot and melted, add the onions and cook until golden, about 4 minutes.
Add the sliced mushrooms and cook until your desired color and texture. (We did 4 minutes.)
Remove the onions and mushrooms to a small bowl.
Place the remaining butter in the pot.
Meanwhile, prepare the pasta according to package directions.
Whisk the flour into the hot, melted butter to form a roux, then whisk in the milk. Season with salt, pepper and thyme and cook for 8 minutes until thickened.
Stir in the chicken broth, pasta, green peas and cooked ham and cook until ham is warmed through.
Set oven to broil.
Pour the pasta mixture into an 8" x 8" casserole dish and sprinkle the grated Parmesan cheese overtop.
Place the casserole dish in the oven and broil until cheese is golden and melted.
Notes
This Recipe Can be Doubled: This is a great recipe for a potluck. Just increase the ingredients proportionately and split it into two baking dishes before baking.
Toaster Oven: You can also use a toaster oven to broil this ham tetrazzini.
Other Veggies: Feel free to switch in and out veggies as you please. If you're not a fan of peas, broccoli florets would also be delicious.