Prep the slow cooker: Lightly butter or spray the inside of a 4 to 6-quart medium slow cooker to prevent the pasta from sticking to the edges.

Mix the cream sauce: Pour the heavy cream directly into the slow cooker. Whisk in the prepared yellow mustard, kosher salt, and black pepper until the liquid is smooth and pale yellow. Stir in 1 cup of the hot water to thin the mixture out.

Coat the noodles: Add the dry wide egg noodles to the slow cooker basin. Gently toss them in the mustard cream liquid, ensuring as many noodles as possible are coated and submerged. If the noodles look extremely dry, sprinkle in up to ½ cup of additional hot water.

Add the butter: Scatter the cut butter pieces evenly over the top of the noodles so they melt down into the sauce as it heats.

Slow cook: Secure the lid and cook on LOW for 1 ½ to 2 hours. Gently stir the noodles every 30 minutes, tucking any dry top noodles back down into the sauce. If the mixture looks too dry before the pasta is tender, stir in a splash of hot water (2 to 4 tablespoons at a time).

Check for doneness: After 1 ½ hours, monitor the pot closely. The dish is done when the noodles are perfectly tender but not mushy, and the liquid has reduced into a glossy, pale yellow coating. It should not look soupy.

Season and rest: Taste the noodles and add a pinch more salt or an extra spoonful of mustard if you prefer a stronger tang. Turn the slow cooker to WARM and let the dish rest for 5 to 10 minutes so the sauce can fully thicken before serving hot.