Add diced potatoes to a large pot of cold water. Cover and bring to a boil. Once boiling cook for 8 to 10 minutes, or until the potatoes are just starting to be fork tender. Drain, transfer to a large mixing bowl and let cool.

Meanwhile, melt 4 tablespoons of butter in a small skillet. Add the shallots and a pinch of kosher salt, sauté until tender and translucent. About 5 minutes. Then add in the garlic and thyme, stir and cook for 1 to 2 minutes.

Sprinkle in the flour, stir and cook 1 to 2 minutes. While stirring, pour in the broth and milk. Heat on medium-low, stirring until thickened. Remove off of the heat and stir in 1 cup sour cream and season with salt and pepper.

Preheat your oven to 350° and lightly spray a 3-1/2 quart baking dish with olive oil spray.

Pour the slightly cooled shallot/sour cream mixture over the cooled potatoes. Add in the grated gruyere and gently toss to coat.

Transfer the potato mixture to your prepared baking dish. Spread evenly and bake on the middle rack of your preheated oven for 45 to 60 minutes or until bubbly and the top is golden brown.

Let the gruyere potatoes cool for 10 to 15 minutes before serving.