Soak the cut potatoes in 1.1 L (4 cups) ice water for 30 minutes. This removes excess surface starch, preventing fries from bonding and helping crispness.
Drain and pat the fries completely dry. Dry surface equals proper browning. If wet, the oil splatters and fries steam rather than crisp.
Heat oil in a deep pot to 150 C (300 F). Blanch fries in batches for 5 to 6 minutes until tender but not colored. The blanched fries will feel soft and slightly translucent.
Remove blanched fries, drain on a wire rack, and cool for 10 minutes. Cooling firms the interior and readies them for final fry.
Increase oil to 190 C (375 F). Fry the cooled fries in batches for 2 to 3 minutes until deep golden and crunchy. They should snap when bent and show a toasted aroma.
Transfer fries to a rack, season immediately with 15 g (1 tbsp) kosher salt and smoked paprika if using. Seasoning adheres best while the fries are hot.
While fries cool slightly, heat a heavy skillet over medium-high. Add sausages and sear for 3 to 4 minutes per side, turning for even color. You should hear a steady sizzle and see a browned crust.
Lower heat to medium, add 15 g (1 tbsp) butter or oil, and continue cooking sausages until internal temperature reaches 71 C (160 F) for pork. The casing should be browned and slightly crisp.
Rest sausages 3 to 5 minutes. Resting lets juices redistribute. Slice on the bias for nice presentation and combine with fries. Garnish with black pepper and parsley.