Prep Vegetables: Peel the potatoes and cut them into rough 1-inch chunks. Peel the carrots and slice them into 1/2-inch rounds or small chunks so they cook at about the same rate as the potatoes.

Load Slow Cooker: Add the potatoes and carrots to the slow cooker in an even layer. Sprinkle the salt evenly over the vegetables.

Add Butter: Dot the top of the potatoes and carrots with 2 tablespoons of the butter, cutting it into small pieces so it melts and runs down between the vegetables as they cook.

Cook: Cover the slow cooker with the lid and cook on HIGH for 3 1/2 to 4 hours, or on LOW for 6 to 7 hours, until the potatoes and carrots are very tender when pierced with a fork.

Mash: When the vegetables are soft, add the remaining 2 tablespoons of butter directly to the slow cooker. Use a potato masher or a sturdy spoon to mash the potatoes and carrots together right in the crock until you reach a fluffy, slightly chunky texture. The mash should be pale yellow streaked with bright orange.

Adjust Consistency: Taste and adjust the salt if needed. If the mash seems too thick, you can mash in a tablespoon or two of hot water from the slow cooker or the kettle to loosen it slightly, keeping the rustic texture.

Serve: Spoon the carrot and potato mash onto a warm plate or into a shallow bowl. Top each serving with a small extra pat of butter so it melts and pools into the warm vegetables before serving.