Using a mandolin, slice the zucchini into long, thin slices.
Spray a 7-inch springform pan with oil and arrange the zucchini in overlapping layers in the bottom of the pan.
Place 1/4 cup of marinara sauce on top of the zucchini and spread evenly.
In a large bowl, mix the onions, garlic, and Italian sausage. Layer the meat on top of the zucchini and spread evenly.
Pour the rest of the marinara sauce and spread it evenly.
Rinse the bowl you used earlier, and mix together the ricotta and mozzarella and 1/4 of a cup of the Parmesan cheese.
Spread the cheese mixture on top of the meat.
Top with the remaining Parmesan cheese.
To recap, the layer are so: zucchini, sauce, meat, sauce, cheese mix, Parmesan cheese.
Cover the pan with foil or a silicone lid.
In the inner liner of the instant pot, place 2 cups of water in the instant pot. Place a steamer rack in the pot. Place the covered lasagna bake on the steamer rack.
Cook on high pressure for 20 minutes. Allow the pot to rest undisturbed for 10 minutes and then release all remaining pressure.
At this point you can either broil it in a hot oven to brown the top, or serve as is by unclasping the sides of the springform pan.
Use a metal springform pan. If you use glass, you will have to adjust the timing on this.
Wrap the bottom of the springform pan in foil if yours leaks. I have linked to this springform pan, but honestly, I have had several of these, some of which have leaked and others haven't. So if your pan leaks, add a sheet of foil at the bottom. You will then also need to increase cook time by 5 minutes or so since the foil will retard heat transfer.
You can either create a sling made from foil that will help you lift out the lasagna afterward or you can use silicone mitts.
I prefer silicone mitts as the regular cloth ones take on the steam from the pressure cooker as you open it, get damp, and then get hot very quickly. The silicone mitts are also easy to throw into the dishwasher for cleaning.
You can choose to broil the top of your lasagna afterward. If you have a Ninja Foodi this is very easy.
You can also do the same thing with a Mealthy Crisp Lid to brown the top of the lasagna.
Due to the moist environment in a Pressure Cooker, you will likely have some condensation on the top of your lasagna. You can either ignore this because after all--deliciousness--or you could cover it with foil to keep the moisture off. By now you know what I'm going to say--if you cover with foil, add a few minutes of cook time!
You could also just dab this moisture off with a paper towel before broiling.
Notice I do NOT ask you to precook the meat. Pressure cooking will cook that meat for you, and here once again, is #ruthlessefficiency at work.
Note that you can reduce carbs even more by using a low-sugar marinara sauce.
You can also cut onions down to 1/2 cup and get even more carb savings.