Preheat oven to 375 degrees and place rack in center position
Start water to boil and cook pasta to package instructions
While water heats, grate cheddar and slice onions
Place milk in a small pot on medium heat to warm
In skillet melt butter on med/high. If not using a skillet melt in a medium sized pot
When butter is melted add a 1/4 cup green onions and sauté for 2 minutes
Add flour and whisk until smooth about one minute. This is a roux. A roux thickens your cheese sauce
Slowly pour warm milk into roux with one hand while whisking with the other
Add 4 cups cheese one handful at a time whisking between additions (reserving one cup of cheese for later)
Whisk until all cheese is melted and sauce is smooth
Season with salt and pepper
When pasta is ready (do not overcook!) add to skillet with cheese sauce and stir until well incorporated. (If baking in a baking dish, pour pasta into baking dish and then pour cheese sauce over)
Top with remaining cheese and bake for 20-25 minutes or until cheese is crispy
Garnish with remaining green onions, serve