Measure the sour cream: Before you begin preparing the potatoes, make sure to measure the sour cream so it can come to room temperature. This will help it blend smoothly into the sauce later.
Boil the potatoes: In a large pot or Dutch oven, add the chopped potatoes and cover them with cold water, ensuring it’s about an inch above the potatoes. Bring it to a boil, then add one tablespoon of salt. Keep boiling until the potatoes are fork-tender, around 4 to 5 minutes after boiling. Drain them in a colander.
Preheat the oven: While the potatoes are cooking, preheat your oven to 350°F. Lightly spray a 9x13 baking dish with nonstick cooking spray and set it aside.
Make the sauce: In a Dutch oven, melt the butter over medium heat. Once melted, whisk in the flour and cook while stirring for about 2 minutes. Reduce the heat to low and gradually whisk in the milk and 1 ½ cups of chicken broth. In a small bowl, whisk the cornstarch with the remaining chicken broth, then add it to the pot along with the Dijon mustard, Worcestershire sauce, chives, chicken bouillon, and spices.
Thicken the sauce: Bring the sauce to a gentle simmer while whisking constantly for about 5 minutes, or until it thickens to your desired consistency.
Add the cheeses: Turn off the heat and whisk in the cheddar cheese a handful at a time until melted, followed by the Gruyere. If the cheese isn't melting smoothly, return the heat to low while whisking. Once melted, stir in the sour cream until well-blended, then gently fold in the cooked potatoes. Adjust seasonings if necessary.
Assemble the dish: Transfer the cheesy potato mixture to the prepared baking dish, spreading it into an even layer. Top with one cup of shredded cheddar for that extra cheesy goodness.
Bake: Cover the casserole with foil and place it in the oven. Bake at 350°F for 25 minutes, until hot and bubbly and the cheese is melted. Let it rest uncovered for about 10 minutes before serving.
Make the panko topping: While the casserole is baking, melt the remaining butter and olive oil together in a skillet over medium heat. Add the panko breadcrumbs and stir to coat. Cook until the crumbs are golden brown, then sprinkle evenly over the baked potatoes.
Garnish and serve: If desired, garnish with fresh parsley before serving for a pop of color. Enjoy!