Preheat the oven to 415°F and line a sheet pan with parchment paper. Peel all the green skin off the zucchini and slice into 1/4 inch slices and salt and pepper well. It's imperative to peel the zucchini, otherwise your sauce will be an ugly green. Bake about 15 minutes until tender.
I cooked the potato in the microwave for about 5 minutes until very soft. Do not boil them as they will get water logged and make the sauce more runny.
Mash the potatoes with a fork and measure out 1 cup. If you followed the correct weight measurement, you should get just about 1 lightly filled cup of mashed potatoes. Update: Using a food processor will make this too gooey, make sure to use just a blender.
Add the zucchini, potato and remaining ingredients (start with 3/4 cup water and add as needed, stopping at 1 cup) to the blender and blend until smooth. Stop and scrape a couple of times from the bottom during. Do not be too eager to add extra water until you've really blended it up a couple of minutes. It will seem too thick at first, but it will start to turn very smooth from all the water from the zucchini. If after blending it's too thick for you, add just a TINY amount just to blend it. If it's a bit thinner than you like, heat on the stove to thicken.
Taste and add any extra lemon juice and/or salt/spices if necessary. Add any desired toppings such as red pepper flakes, chopped tomatoes, sliced jalapenos. I like a kick of heat to my cheese so I added red pepper flakes.
To make it as a mac 'n' cheese, add cheese sauce to cooked pasta. You can either warm the cheese by running it awhile in the blender or gently warming over the stove. It gives such an incredible cheesy, thick and stick-to-pasta result.