Add 1 teaspoon salt to 1 quart water in a medium size pot.
Bring to a boil, add pasta and cook according to the package directions to the al dente stage. Drain and set aside.
Meanwhile, cook bacon. Crumble Set aside.
In the skillet that bacon was cooked, melt butter on medium heat.
Add onions and cook 2 minutes.
Add 2 Tablespoon flour and cook 1 minute.
Add half and half and whisk to prevent clumps.
Add Cajun seasoning and crawfish tails and stir 2 to 3 minutes or until crawfish are heated through.
Add cheeses and stir until melted.
Fold pasta and half of the bacon into cheese sauce, careful not to break pasta.
I transfered to smaller serving dishes, but you could leave in one large skillet. Top with remaining bacon and bread crumbs. With oven on broil, place in oven and toast, watching carefully, until breadcrumbs are golden.
Remove garnish with green onions slices. Allow to cool 5 minutes before serving.