Thinly slice the carrots crosswise 1/4 inch thick.
In a medium saucepan, place carrots, water, butter, and salt. Cook over high heat just until it starts to boil. Reduce the heat to low, cover, and continue to cook until the carrots are tender and soft - about 20 minutes. Make sure to cover the carrots because we don’t want all the water to evaporate. Remember, there’s butter and salt in the water and that is flavor.
Once the carrots are soft and tender enough that you can mash with a fork, transfer them to a blender with a slotted spoon. There should be at least 3/4 cup of liquid left in the pan. Start with adding 1/2 cup of that to the blender along with 1 tablespoon of the lemon juice.
Blend until smooth. If you need more liquid to get it going, add the remaining cooking liquid. Taste and season with salt. Add more lemon juice to your taste. You'll find a difference in acidity between bottled lemon juice and fresh lemon.
Spoon onto serving plate alongside whatever it is you’re serving. The consistency is smooth, but still thick enough to be eaten with a fork. If you want to be fancy, scoop the carrot puree and spread with a spoon with a serving of meat, poultry, or fish on top.
If the carrot puree is too runny - simply place the puree back in the pan and cook over medium heat until some of the water evaporates. Once you can see the bottom of the pan when a spoon is scraped across, it’s done.