Preheat your oven to 325°F (163°C) and prepare a 9x13-inch casserole dish by lightly greasing it.

In a medium saucepan, combine the frozen mixed vegetables, broth, kosher salt, and garlic powder. Steam over medium heat for about 3-4 minutes until fork-tender.

In another saucepan, melt the butter or olive oil, then stir in the flour. Cook for 1 minute, then whisk in the milk and reserved vegetable liquid. Bring to a gentle boil until thickened.

Lower the heat and add white wine, thyme, nutmeg, Parmesan, and Gruyere cheeses to the sauce. Stir until the cheeses are melted and the sauce is creamy.

Pour the Mornay sauce over the steamed vegetables in the casserole dish and mix gently to coat evenly.

In a small bowl, combine breadcrumbs with olive oil until coated, then spread over the casserole.

Cover with foil and bake for 30 minutes. Remove the foil for the last 15 minutes to brown the top.

Let sit for about 10 minutes after baking before serving.