To prick or not to prick -- the question facing anyone who cooks cased sausages -- can be answered with a resounding "no." The next common dilemma -- cooking fast over high heat or cooking slow over low -- can't be answered as easily, because both techniques work in a cast-iron skillet. Cast-iron's claims to fame are its even heat distribution and retention, which means that you can cook in cast-iron for a prolonged period without much fluctuation in surface temperature. That gives you two choices when cooking sausages in cast-iron, both of which work equally well: slow-frying and poaching, then searing.
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Take the sausages out of the refrigerator and let them approach room temperature, about 15 minutes.
Add a tablespoon or so of butter to the cast-iron skillet and warm it over low heat for 5 minutes. Pat the surface moisture from the sausages with a paper towel and place them in the pan, spacing each at least 1/2 inch away from the next.
Fry the sausage until they develop a crisp, brown crust, about 35 to 40 minutes, turning every 5 or 10 minutes. Check the internal temperature of a sausage in the center using an instant-read thermometer; sausage needs to cook to 165 degrees Fahrenheit.
Take the sausages out of the refrigerator 15 to 30 minutes before you want to cook them. Add about 1 inch of water to the cast-iron pan and set it over medium heat.
Heat the water until it reaches 150 F. Adjust the heat on the stove as needed to keep the water temperature between 140 and 150 F.
Add the sausages and poach them for 20 minutes. Transfer the sausages to a plate lined with paper towels and pat them dry.
Pour out the water from the cast-iron pan and wipe it dry. Add a tablespoon or so of butter and set it over medium heat; heat the butter until it starts to caramelize, about 4 to 5 minutes.
Add the sausages to the pan and fry them until crisp and crusty on the outside, about 3 to 4 minutes.