Freeze the lobster for 1 hour. Remove the 8 legs and antenna with kitchen scissors
Pop into a 7 litre pot with heavily salted boiling water. Cook 6 minutes. Add the legs in at the 3 minute mark.
Remove and sit on the bench. Set on its side and let cool. Wrap the tail around a small saucepan or similar so it doesn't curl up into a tight ball while cooling.
Cut in half lengthways and remove the meat. Sharp knife and kitchen scissors in combo work well.
Clean out the shells. Set cut side down and let dry on paper towels.
Cut the meat into chunky natural pieces along the break lines. Set aside.

Melt the butter and add the flour to make a roux. Add 100ml milk and cook till stiff. Remove from the heat. Add the chopped cheese into the mix to melt. Add the sherry and cream and heat till stiff again. Add just enough milk to make a smooth, really thick mornay sauce.

Add the lobster meat and gently stir through till coated.

Use some of the mashed potato in dabs to support and position the shells on the plate.
Spoon equal amounts of lobster into each shell. Spoon over extra mornay sauce.
Grill 4 minutes till browned and bubbling.

Serve with creamy Mashed potatoes, Asparagus Spears and Mango Pumpkin puree. 
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To make the Mango Pumpkin puree:: Roast some pumpkin for an hour and let cool. Blend the pumpkin, mango flesh, some lemon juice and cream. Heat in a microwave just before service.

Actually prefer to use a shell shaped dish. It is easier and it ups the dish in a couple of ways. First you can heat the dish to quite hot then add the lobster mornay and grill. This puts more heat evenly through the mornay and retains the heat longer. Second add sliced lemons around the edge before grilling. This does make it a better dish.

You want as creamy a sauce as possible. Too much butter and cream is barely enough. During the cooking this can cause the fat to seperate out of the sauce. No problem. If excessive mop up with a paper towel. Then add a small amount of milk to bring back together. You know you have as much butter in the sauce as is possible when this happens. I do this with my sauce everytime as a deliberate act.

A 1.2kg lobster boiled for 6 minutes is just undercooked. Grilling will complete the cooking process perfectly.

1.25 kg Live Tasmanian Lobster

80g Salted Butter
75g Flour
2 cheese slices (processed)
50ml sherry
100ml cream
225ml milk or so

Mashed potato
Asparagus spears
Mango and pumpkin puree
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A good trick, told to me by a chef I went Kart racing with, is to add up to 50% Moreton Bay bug (Balmain bug) meat at much less costs and recycle some shells. Or serve in a shell shaped dish.

You know even though this is cheating done in a restaurant, at home level it is a pretty good idea. It works really well. Lobster and Bug mornay is the correct description.
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Lobster Newberg and Thermidor are very similar dishes. They use egg instead of flour and add more spice, cayenne and nutmeg, and brandy as well as sherry. I don't like it. Australian lobster is the best eating lobster on the planet and the less you do the better it is. A very mild flavoured yet rich mornay sauce is perfect.

On lessor lobster more introduced flavours might help.

Lobster Mornay was the most luxurious restaurant dish of the 1960's. An expensive premium ingredient.
During the 80,s it became more usual (better value) and was on every fine dining restaurants menu. Lobster wasn't anywhere near as expensive relatively at this time and I would cook myself one at least every month.  Then as exports increased Tasmanian Lobster was considered the best eating lobster on the planet. Asia and China wanted our lobsters. Prices rose. It got so bad that regardless of cost in 2017 I decided to cook the family lobster for Good Friday. Problem was there were no Australian whole lobsters to buy. Had to settle for Brazilian Lobster tails and do a rissotto. Just ridiculous. Covid provided some relief on lobster and other seafood prices for a while.

Lobster is a great ingredient and not much improves it. Butter, cream and cheese are the main things. I think that sherry is the most complimentary flavour to add. That's it. This generous version with live Tasmanian lobster is something you won't see. A frozen 600g lobster is still good. However, far too much of amazing lobster is barely enough. Experience it at least once. It's worth it.