Directions:

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 for about 15 minutes until tender.

Meanwhile, cook your potatoes either by roasting, steaming or in the microwave. Since I wanted a fast dinner, I just cooked them 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. I left the skins on since I have a Vitamix, but if you are concerned about getting the sauce smooth, peel them before cooking or blending.

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. Add them to a high speed blender. Update: Using a food processor will make this too gooey, make sure to use just a blender.
Add the zucchini, potato and the 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 on high to get it completely smooth. 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, you can reheat it over the stove to thicken it up some.

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 but will totally blend up some jalapenos next time in the sauce. Sauce makes 2 1/2 cups.
To make it as a mac 'n' cheese, just simply boil your pasta and pour as much (a lot!) cheese sauce over the 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.