Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large skillet, heat the olive oil over medium heat. Add the chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 3: Add the thinly sliced zucchini to the skillet. Pour in the chicken or vegetable broth. Season with oregano, thyme, salt, and pepper. Cook, stirring occasionally, for 5-7 minutes, until the zucchini starts to soften but is still slightly firm.
Step 4: Transfer the zucchini mixture to the prepared baking dish, spreading it evenly.
Step 5: In a small bowl, combine the panko breadcrumbs and grated Parmesan cheese. Sprinkle this mixture over the zucchini.
Step 6: Evenly sprinkle the shredded mozzarella cheese over the breadcrumb mixture.
Step 7: Bake for 25-30 minutes, or until the zucchini is tender and the cheese is melted, bubbly, and golden brown. If needed, you can broil for the last 1-2 minutes for extra browning, watching carefully to prevent burning.
Step 8: Remove from the oven and let rest for 5 minutes before serving. Garnish with fresh chopped parsley if desired.