Preheat your oven to 400°F and lightly coat a 9x13 inch baking dish with cooking spray. While the oven heats, prepare all your ingredients: finely dice the onion into 1/4-inch pieces, slice the yellow squash and zucchini into 1/4-inch rounds, mince the garlic, measure out the kale, and beat the egg in a small bowl. Having everything prepped and ready will make the cooking process smooth and prevent overcooking any components.

Heat the olive oil in a large pot over medium heat, then add the diced onion and sauté for about 5 minutes until softened and fragrant. Add the garlic, kale, salt, and black pepper, stirring constantly for about 1 minute to wilt the kale slightly. Pour in the vegetable broth and simmer for 2-3 minutes until the broth has mostly reduced. This gentle cooking builds a flavorful foundation without overcooking the delicate kale.

Add the yellow squash and zucchini slices to the pot along with a pinch of additional salt and pepper. Cook for 5-8 minutes, stirring occasionally, until the vegetables are just tender but still hold their shape—you want them cooked through but not mushy. Remove from heat and stir in the oregano, basil, lemon zest, and red pepper flakes. I prefer to add herbs at the end of cooking to preserve their bright, fresh flavors rather than cooking them down for too long.

In a large mixing bowl, combine the cooked vegetable mixture from Step 3 with the Greek yogurt, cooked quinoa, mozzarella, parmesan, and 1 cup of panko. Stir until everything is evenly distributed and well combined—the Greek yogurt and cheese will coat all the vegetables and grains. Add the beaten egg and fold it in gently until the mixture is homogeneous. The egg acts as a binder to help the casserole hold together when baked.

Transfer the entire mixture from Step 4 into your prepared 9x13 inch baking dish, spreading it evenly with a spatula. In a small bowl, combine the 3 tablespoons of panko and 2 tablespoons of parmesan, then sprinkle this mixture evenly over the top of the casserole. I like to gently press the topping down slightly so it gets golden and crispy during baking rather than staying loose and fluffy.

Bake in your preheated 400°F oven for 30-35 minutes, until the top is golden brown and the casserole is heated through. The casserole is ready when the top is lightly browned and a knife inserted into the center comes out hot. Let it rest for 5 minutes before serving—this allows the casserole to set and makes it easier to portion and serve.