1. Set a large dutch oven or stockpot with 4 quarts of water over high heat on the stove for the pasta.
2. Pat shrimp dry with paper towels and sprinkle with a pinch or two of salt, a pinch of fresh ground pepper and the sugar to season it. In a 12 inch skillet, melt 2 tablespoons of butter over medium high heat until it's just beginning to turn brown. Add the shrimp, and arrange them quickly so they are in a single layer in the pan. Cook for 1 minute without moving them. Stir and continue to cook until they are spotty brown and just pink at the edges, about 30 seconds. Transfer the shrimp to a bowl and cover tightly with foil. Set aside.
3. Now add 2 more tablespoons of butter to the skillet until melted. Add the onion and a pinch of salt. Cook for about 5 or 6 minutes, and add the garlic and red pepper flakes, stir and cook for about 30 seconds, and add in 1/2 cup of broth. It will sizzle quite a bit, that's normal. As it boils, scrape any brown bits off the bottom of the pan. When it's nearly evaporated (about 2 minutes) add in the rest of the broth and the dissolved cornstarch mixture and simmer until slightly thickened, another 2 minutes. Remove the skillet from heat and set aside.
4. The pot of water for the pasta should be boiling at this point. Add the linguine and 1 tablespoon of salt to the water and cook, stirring often, until the pasta is al-dente. (There should be directions on the package for this.) You can take this time to squeeze the juice from the lemon and mince the parsley, stopping occasionally to stir the pasta. When the pasta is done, scoop out about 1/2 cup of pasta water and set aside. Drain the pasta and return it to the pot.
5. Add in the remaining two tablespoons of butter, lemon juice, parsley, sauce and reserved shrimp (including any shrimp juice that may have accumulated) into the linguine. Toss to combine. Season to taste with additional salt and pepper, adding the pasta water if needed to loosen the sauce. Serve immediately.