Preheat your oven to 375°F (190°C).
In a large skillet, heat the olive oil over medium heat. Add the onion and garlic, sautéing until they become translucent.
Add the sliced summer squash to the skillet and cook for about 5 minutes until they start to soften.
In a large bowl, whisk together the eggs and milk. Stir in the shredded cheese and chopped herbs.
Season the egg mixture with salt and pepper to taste.
Add the cooked squash mixture to the bowl and stir until everything is well combined.
Pour the mixture into a greased glass baking pan.
In a small bowl, mix the breadcrumbs with the grated Parmesan cheese. Sprinkle this mixture evenly over the top of the casserole.
Bake in the preheated oven for 30-35 minutes or until the casserole is set and the top is golden brown.
Allow the casserole to cool for a few minutes before serving.
Variations & Tips
For a bit of spice, add a pinch of red pepper flakes to the egg mixture. If you have picky eaters, consider blending the squash into the egg mixture for a smoother texture. You can also swap out the summer squash for zucchini or a mix of both. For a gluten-free version, use gluten-free breadcrumbs or omit them entirely.