Boil a large pot of water and cook pasta until al dente.
Drain, reserving the pasta water for later.
Melt 4 Tbsp butter in a large pan over medium heat. Add in flour and whisk to combine and cook until it turns light brown.
Add in milk, whisking as you pour. Stir until smooth.
Add in seasoning and cook until sauce thickens.
Add in cheeses, one at a time and stir until each cheese is melted before adding next cheese.
Add in pasta. Use remaining pasta water to thin sauce if it has thickened too much.
Place mixture into greased dish.
Melt remaining 2 Tbsp butter and add the breadcrumbs, Parmesan and a pinch of salt. Stir to combine
Sprinkle over top of pasta and bake until browned and bubbly. About 15-20 minutes.
Garnish with sliced green onions.
Notes
I used one of my favorite cheese combos, sharp white cheddar cheese, gruyere cheese and Monterey jack cheese. (Sometimes I use pepper jack cheese instead of the Monterey jack) The combination not only has great taste but it is nice and creamy.If you can't find sharp white cheddar, use the sharp yellow cheddar.