Brown the beef in a skillet over medium high heat for about 5 minutes. Add onion and cook 3 minutes, then stir in garlic and tomato paste for 30 seconds. Add crushed tomatoes, oregano, red pepper flakes, and pepper. Simmer uncovered 15 to 20 minutes until thick. Season with salt. Preheat oven to 375°F (190°C).

Combine ricotta, egg, ¼ cup Parmesan, 1 cup mozzarella, and a pinch of salt and pepper. Mix until smooth.

Slice zucchini lengthwise into ⅛ inch strips. Lay on towels, sprinkle with 1 tbsp kosher salt, and let sit 10 minutes. Pat dry without rinsing.

Spread 1 cup meat sauce in a 9x13 inch dish. Layer zucchini strips overlapping by ¼ inch, then half the ricotta mixture. Repeat with sauce, zucchini, and remaining ricotta. Add another layer of sauce, the last zucchini layer, and the rest of the sauce. Top with remaining 1 cup mozzarella and remaining Parmesan.

Bake at 375°F for 35 to 40 minutes until bubbly and golden. Broil 1 to 2 minutes for extra browning if desired. Let rest 10 minutes.

Cut into squares and serve warm.