Adapted from Magnolia Table
Preheat the oven to 250°F.
Spread the torn bread on a sheet pan and dry in the oven for 45 minutes until crunchy and golden brown. Set aside.
In a large skillet, heat the oil and butter over medium heat. Add the sliced zucchini and squash and sauté until golden brown, for about 15 to 18 minutes.
Transfer to a plate or sheet pan to cool completely, for 20 to 30 minutes.
Grease a 9 × 13-inch baking dish with butter.
In a large bowl, mix the bread, Cheddar, Fontina, Parmesan, sautéed zucchini/squash mixture, and garlic salt until well combined.
In a separate large bowl, whisk together the eggs, milk, salt, and pepper until smooth. Pour the egg mixture over the zucchini and bread mixture and stir until the bread is well coated.
Spoon the mixture into the prepared baking dish. Cover the dish tightly with plastic wrap and refrigerate for at least 1 hour or up to overnight.
Preheat the oven to 350°F.
Remove the plastic wrap and bake the strata until the casserole is set and the top is nicely browned, for 35 to 40 minutes.
Sprinkle with the chopped fresh parsley and serve while still hot.
Store in an airtight container in the refrigerator for up to 3 days.
Garlic Salt: If you don't keep a stash of garlic salt in your spice cabinet, you can easily create your own supply. Use the formula of three parts salt mixed with one part garlic powder, mix well and you're all set.