Slice the zucchini into 1/2-inch rounds. Put the rounds in a sieve or colander over the sink or a bowl and sprinkle with salt, tossing to coat.

Let the salted zucchini sit for 15 minutes to draw out excess moisture.

Rinse the zucchini well under cold water to remove the excess salt, then pat it dry with paper towels.

Heat the olive oil in a large skillet over medium-high heat. Add the onion and cook until it begins to soften, about 2 minutes.

Add the garlic and ground beef, breaking the beef up with a spoon, and cook, stirring often, until it is browned with no trace of pink, about 5 minutes. Check that the beef reaches 160°F on a food thermometer.

Stir in the drained tomatoes, Italian seasoning, kosher salt and black pepper. Remove the skillet from the heat.

Heat the oven to 350°F.

In an 8x8-inch baking dish, spread half of the beef mixture. Cover it with half of the zucchini slices, then half of the mozzarella. Repeat with the rest of the beef mixture, zucchini and mozzarella.

Bake for 20 to 25 minutes, until the cheese is melted and beginning to brown and the center reaches 165°F on a food thermometer.

Let the casserole rest for 10 minutes before serving.