Step 1

Preheat the oven to 400 degrees F. Coat a 2 ½-quart baking dish (such as an 8x10-inch casserole dish) with nonstick spray.

Step 2

Slice the zucchini crosswise into ¼-inch thick rounds (I find this quickest with a mandoline). Place in a large colander. Sprinkle with 1/2 teaspoon of the salt. Toss to coat the zucchini with the salt, making sure to separate the slices if needed. Place the colander over a large bowl (or in the sink, but don't forget about it and accidentally turn on the water). Let drain at least 20 minutes.

Step 3

Cut the onion in half through the stem ends, peel, and cut each half into thin, long strips.

Step 4

In a Dutch oven or similar large, sturdy sauté pan, melt 2 tablespoons of the butter over medium heat. Add the onion, black pepper, and remaining 1/4 teaspoon salt. Sauté until the onion is beginning to soften and turn translucent, about 5 minutes. Stir in the garlic and let cook 30 seconds.

Step 5

Sprinkle the flour over the top. Cook, stirring constantly, for 1 minute, until all of the flour turns golden and no white bits remain.

Step 6

Slowly add the broth and milk a few splashes at a time, stirring between each addition to prevent lumps from forming. Increase the heat to medium high. Cook and stir, allowing the sauce to bubble, running a wooden spoon or spatula along the bottom of the pan. Continue cooking and stirring until the sauce reduces and thickens to resemble a very thick, creamy gravy, about 3 to 4 minutes. Remove from the heat.

Step 7

Shake the colander to remove as much water as possible, then spread the zucchini onto paper towels or a clean kitchen towel. Blot dry, gently pressing out as much moisture as possible.

Step 8

Stir the Greek yogurt into the sauce.

Step 9

Add the cheddar cheese a few handfuls at a time, stirring between each addition to ensure it melts smoothly.

Step 10

Stir in 2 tablespoons of the Parmesan and the thyme.

Step 11

Add the zucchini by handfuls, stirring as you go to coat the slices evenly.

Step 12

Stir in 1/4 cup panko, then transfer the mixture to the baking dish.

Step 13

In a small bowl, melt the remaining 1 tablespoon butter. Add the remaining 1/4 cup panko and remaining 2 tablespoons Parmesan, stirring to combine. Sprinkle evenly over the casserole.

Step 14

Bake the zucchini casserole until the filling is hot and bubbly and the zucchini is tender but not mushy, 30 to 35 minutes. If desired, switch the oven to broil to brown the top (watch carefully! You will only need a minute or so). Let rest at least 5 minutes, then enjoy.