Zucchini, Mushroom, and Spinach Casserole

This vegetable casserole layers tender zucchini with savory mushrooms, wilted spinach, mozzarella, and a crisp panko top. Salting and blotting the zucchini first limits excess water, while cooking the mushrooms until their liquid evaporates keeps the finished layers cohesive rather than soupy.

Yield: 6 side-dish servings
Prep time: 25 minutes
Cook time: 40 minutes
Total time: 1 hour 5 minutes

Equipment

• 9-inch square baking dish
• Large rimmed tray or cutting board
• Large skillet
• Colander
• Chef’s knife and cutting board
• Paper towels or a clean kitchen towel

Ingredients

• 2 medium zucchini, about 1 pound total, sliced into 1/4-inch rounds
• 3/4 teaspoon kosher salt, divided
• 1 tablespoon olive oil, plus 2 teaspoons for the topping
• 12 ounces cremini mushrooms, sliced 1/4 inch thick
• 2 garlic cloves, minced
• 1/2 teaspoon onion powder
• 1/4 teaspoon black pepper
• 5 ounces baby spinach
• 1 teaspoon finely grated lemon zest
• 1 cup shredded low-moisture mozzarella, divided
• 1/2 cup panko breadcrumbs
• Cooking spray

Method

1. Heat the oven to 375°F. Coat the baking dish lightly with cooking spray.
2. Spread the zucchini on the tray and season with 1/2 teaspoon salt. Rest for 15 minutes, then blot both sides thoroughly. This draws out surface moisture before baking.
3. Meanwhile, heat 1 tablespoon oil in the skillet over medium-high heat. Add the mushrooms and cook for 7 to 9 minutes, stirring occasionally, until browned and the pan looks dry. Add the garlic, onion powder, pepper, and remaining 1/4 teaspoon salt; stir for 30 seconds.
4. Add the spinach by handfuls. Cook for 2 to 3 minutes, tossing until fully wilted and no liquid pools in the skillet. Remove from the heat and stir in the lemon zest. Let cool for 5 minutes.
5. Arrange half the zucchini in an overlapping layer. Top with half the mushroom mixture and 1/3 cup mozzarella. Repeat with the remaining zucchini, mushroom mixture, and another 1/3 cup mozzarella.
6. Combine the panko, remaining 1/3 cup mozzarella, and 2 teaspoons oil. Scatter evenly over the casserole.
7. Bake uncovered for 30 to 35 minutes, until bubbling at the edges, the zucchini is fork-tender, and the topping is golden. If the top browns early, cover loosely with foil. Rest for 10 minutes before cutting so released juices can settle.

Ingredient Functions and Success Cues

Low-moisture mozzarella melts without adding the water of fresh mozzarella. Panko supplies a contrasting crust, and lemon zest brightens the earthy mushrooms without introducing extra liquid. The vegetable filling should look dry before layering; a wet skillet predicts a watery casserole.

Troubleshooting and Substitutions

If liquid collects after baking, extend the uncovered bake by 5 minutes and allow the full rest. Yellow summer squash may replace zucchini. Button mushrooms work in place of cremini, and kale can replace spinach if its tough stems are removed and the leaves cook until tender. Gluten-free panko is a direct substitute.

Storage

Cool within 2 hours. Refrigerate in a covered container for up to 4 days. Reheat portions to 165°F in a 350°F oven or microwave. Freeze for up to 2 months; thaw overnight in the refrigerator, although the zucchini will soften.