Slice the summer squash into thin rounds using a sharp knife, feeling the slight resistance as you cut through the firm flesh. Place the slices in a bowl and sprinkle generously with salt, then let sit for about 5 minutes to draw out excess moisture.
Meanwhile, finely chop the onion and grate the cheese if not pre-shredded. Set these aside for later.
After the squash has rested, use a clean towel or paper towels to gently pat the slices dry, removing any excess moisture and salt.
Heat a large skillet over medium heat and add the butter. Once melted and bubbling, add the chopped onion and sauté until fragrant and translucent, about 2-3 minutes.
Add the sliced squash to the skillet and cook, stirring occasionally, until they soften and turn a light golden color—about 5-7 minutes. You should hear a gentle sizzle and smell the sweet, green aroma of the squash.
Season the cooked squash with salt, pepper, and chopped herbs, then stir to combine. Remove from heat and transfer to a baking dish.
Sprinkle the shredded cheese evenly over the squash, then top with the panko breadcrumbs for a crunchy finish.
Place the baking dish in a preheated oven at 375°F (190°C) and bake for about 10-15 minutes, until the top is golden brown and bubbly.
Remove from the oven and let sit for a few minutes to settle. The casserole should be hot, with a crispy, golden topping and tender, fragrant squash underneath.
Serve warm, enjoying the crisp topping with the soft, flavorful squash inside. Garnish with additional herbs if desired.
