Boil large pot of water, and once boiling add salt and cook 12 oz. pasta.
In large saucepan, melt 1/2 stick Butter over medium heat. Add to it 1 thinly sliced Medium Onion. Sweat these for a few minutes. Add 2 cloves of pressed Garlic and 8 oz clean thinkly sliced Button Mushrooms. Let cook for a few minutes stirring often. Sprinkle 2 T. Flour onto mixture and continue stirring for a few minutes. Add a 1/4 c. White Wine or a few T. of Dry Vermouth. Once it begins to bubble, add 14 1/2 oz. Chicken Broth and 1/4 C. of 1/2 and 1/2 and continue stirring. (You can substitute these 2 ingredients for 1 small can Cream of Mushroom Soup + milk). Add 1 1/2 - 2 C. Crawfish Tails. Adjust sauce thickness using either milk or 1/2 and 1/2. Let sauce bubble gently. Season to your taste with Tony Chachere's, red pepper flakes, S&P.
By now the pasta should be done al dente. Drain and place back in pot and add a very small amount of olive oil, only enough so the pasta doesn't stick together. Immediately place pasta in a platter. Top with crawfish sauce. Garnish with thinly sliced scallions. Serve immediately.