Put your lobster tails in a large pot of boiling water. Cook them on boil for 9 to 11 minutes, depending on how large the lobster is. (NOTE: If you do not want your lobster to curl during this process, poke a long wooden shish kebob skewer through each lobster from end to end. These will keep the lobster straight while it cooks but the skewers are not mandatory.)
Cool your lobster to the touch. BEFORE you toss out the cooking water, reserve 5 cups of it and set it aside.
When the lobster is cool enough to handle, take out the skewers (if you?ve used them) and remove the meat. If they are fully cooked, you should be able to brace the tail at the back and insert a fork at a 45-degree angle into the meat toward the back and gently pull up on the meat and it will come out in one piece. If you?re not successful with that this the first time, don?t worry. Just take it out whatever way you can and serve chunks of lobster instead of slicing it into medallions. If you see pink, transparent meat, your lobster is not done. Boil it some more. The meat should be white all the way through. Get all of the meat out so you don?t waste any of it. SAVE the tail shells. Set aside the lobster meat or wrap it and chill it until ready to serve.
In a large saucepan, melt two tablespoons of butter on Medium heat. Add the onion, turmeric, cumin and garlic. Saut? to loosen the onion. Then add the tomato paste, the Calvados and the shells along with the cooking liquid that you set aside. You are going to bring the mixture to a boil and then add the white wine and the Italian seasoning. Simmer the mixture for about 30 minutes (or more) until it?s reduced to 2 cups of sauce. Swirl the tails and solids around so you can estimate how much liquid is in there. You have to do this by eye. Strain all of this liquid into a bowl. Be sure to tip the lobster tails so you get every bit of juice out. Let it all sit in the strainer for a few minutes to be sure you get all of the juice. You can cover this liquid and put it in the refrigerator to cool and then finish up the sauce when you are going to serve dinner. Just make sure it?s the same day.
When you want to serve the dish:
Turn your oven on to 400 degrees and put your 4 slices of bread on a cookie sheet. You want to warm the bread and lightly toast it. This is accomplished easily with the oven method. Keep an eye on it after just a minute or two so it doesn?t burn.
While the oven is heating, stir the flour and water together in a small container to get the lumps out of the flour. Then add it to your strained lobster sauce and heat. Stir constantly and heat this mixture on Medium. It will thicken fairly quickly. Turn off or keep on very Low so it doesn?t continue to thicken. (You can always turn it back on to heat it up.)
On a cutting board, slice your lobster into quarter-inch medallions or cut into chunks, depending on how easily the meat came out of the shells. Heat the other two tablespoons of butter on Medium High and put in your lobster meat. Since it?s already cooked, you are just heating this through. It won?t take more than a minute or two.
Arrange your toasted crostini (bread slices) on plates, top with lobster and then drizzle on your sauce. Sprinkle your green onions around the plate for color.