Preheat your oven to 375°F (190°C). In a large skillet, heat olive oil over medium heat and sauté diced onion, chopped carrots, and celery for 5-7 minutes until softened. Add minced garlic, broccoli, and cauliflower, stirring for another 5-7 minutes until everything is tender-crisp. Fold in the frozen peas and season with salt and pepper before setting aside.

In the same skillet, melt butter over medium-low heat. Whisk in flour to create a roux, cooking for 1-2 minutes until lightly golden. Gradually pour in milk and vegetable broth while whisking. Allow the mixture to thicken for about 5-7 minutes; it should be creamy and smooth. Stir in Dijon mustard, nutmeg, cream cheese, and shredded cheddar until fully incorporated.

Once your creamy sauce is ready, fold the sautéed vegetables into the cheese sauce until coated. Grease a 9x13 inch baking dish, then pour the vegetable-sauce mixture evenly into the dish. Smooth the top.

In a small bowl, combine panko breadcrumbs with grated Parmesan and melted butter. Mix until the breadcrumbs are evenly coated. Sprinkle the topping over the casserole.

Place the casserole in the preheated oven and bake for 30-35 minutes until the top is golden brown and the casserole is bubbling. Let the dish rest for 5-10 minutes before serving.