Season the shrimp with salt and add them to the pan in one layer. Increase the heat to medium and sauté until the shrimp begin to color, about 1 minute. Flip the shrimp over, then add the lemon juice and mustard to the pan. Simmer for 20 seconds, then immediately reduce the heat to low. Add 1 tablespoon of the butter and swirl the pan to form a sauce. Continue to swirl the pan and add the remaining butter, 1 tablespoon at a time, until each tablespoon melts, to create a creamy, emulsified sauce, about 2 minutes.