Grate zucchini on the large holes of a box grater. Toss with 1 teaspoon kosher salt in a colander and let drain for 15 minutes. Squeeze handfuls firmly to extract as much liquid as possible - this is crucial for a casserole that holds together instead of swimming in water.

Heat olive oil in a large skillet over medium-high heat. Add squeezed zucchini and cook, stirring occasionally, until it stops releasing liquid and just starts to turn golden in spots, about 8 minutes. Add minced garlic and cook 1 minute more until fragrant. Transfer to a bowl.

In the same skillet, cook Italian sausage over medium-high heat, breaking it up with a wooden spoon, until browned and cooked through, about 6 minutes. Drain excess fat if needed.

Preheat oven to 400 degrees F. Stir pizza sauce, oregano, and red pepper flakes into the cooked sausage. Taste and add salt if your sauce needs it.

Spread half the zucchini mixture in a 9x13 inch baking dish. Top with half the sausage-sauce mixture and 1 cup mozzarella. Repeat layers. Finish with remaining 1 cup mozzarella and all the Parmesan.

Bake uncovered until cheese is melted and spotty golden brown, and the edges are bubbling, 25 to 30 minutes. Let rest 10 minutes before serving - this helps it set so you can cut clean squares. Scatter fresh basil over top.