Preheat the oven to 400 degrees Fahrenheit. Butter a 3 quart (9x13 inch) baking dish and set aside.
Cut the russet potatoes in half crosswise and add to a large pot of water. Bring to a boil and cook until the potatoes are just tender when pierced with a fork, about 20 minutes. When they are fork tender but still have some resistance, scoop them out of the pot with a slotted spoon and place on a plate to cool.
Meanwhile, place the cauliflower florets in a small pot fitted with a steamer basket. Add an inch of water to the bottom of the pot, cover, and bring to a boil. Steam the cauliflower until just barely tender, about 3-5 minutes. Remove the cauliflower from the steamer basket and spread out on a plate or tray to cool.
When the potatoes are cool enough to handle, carefully peel off the potato skins and discard. Cut the potatoes into a 1-inch dice and set aside.
In a small nonstick skillet, melt 4 tablespoons of unsalted butter. Add the panko breadcrumbs and cook over medium-low heat until golden. Stir in the garlic powder, salt, and thyme and remove from the heat.
In a large nonstick skillet, melt 4 tablespoons of unsalted butter over low heat. Add the 1/4 cup flour and whisk until smooth. Keep the heat low to prevent the butter from browning or the flour from burning and cook until the roux is bubbly and barely a light golden color.
Whisk in the heavy cream and milk. Continue whisking over low heat until the sauce is thickened and coats the whisk when removed from the pan. This shouldn't take more than 5 minutes. Stir in 1 teaspoon kosher salt and 1 teaspoon ground black pepper and remove from the heat.
Place the cubed potatoes and cauliflower in a large mixing bowl. Pour the creamy white sauce over the vegetables and toss with a soft spatula until evenly coated. Dump the creamy vegetable mixture into the buttered casserole dish and shake to spread out the mixture evenly. Sprinkle with the Gruyere cheese followed by the Parmesan cheese, and finally, top with the breadcrumb mixture.
Bake uncovered in the oven for about 20 minutes, or until golden and bubbly. Let rest for a few minutes and serve.
Cheesy Potato and Cauliflower Au Gratin by dietitian Erica Julson (https://www.ericajulson.com/cheesy-potato-cauliflower-au-gratin/)