FIRST: Boil the lobster tails and set aside after shells are removed. Cut into chunks. Also, I save the two of the shells and put it into the sauce after it's mixed while the pasta is boiling in the water, just for added fish flavor. You don't have to do this if you don't want to.
SECOND: In a medium size pot, add milk and flour and peeled onion and leave whole. On very low heat mix together until warm and starts to thicken. Add the cheddar cheese chunks stir until melted. Add 2 large amount sharp shredded cheese and continue to stir until all is melted. Always keep on low heat, you don't want it to stick. Also add the one stick of butter, either in the beginning or the end, it doesn't make any difference. When all is blended, remove from heat and set aside.
THIRD: Bring salted water to a boil and make your pasta. Remove to a 13x9 or a deep baking dish. Remove the onion from the cheese sauce, put all into the pasta, mix gently. Add the chopped lobster tails. I chop my tails in large chunks because we like to see the lobster and taste the chunks. Mix all through. Top with the 1 package of shredded cheese and BAKE 350 FOR 1 HOUR. REMOVE AND SERVE....ENJOY.