Preheat your oven to 400°F and lightly butter a 9x13 inch baking dish. While the oven heats, peel and slice the potatoes into 1/8-inch rounds—I recommend using a mandoline slicer for consistent thickness, which ensures even cooking. Finely mince the garlic cloves, dice the onion into small pieces, and shred the gruyere and parmesan cheeses fresh (pre-shredded cheese contains anti-caking agents that prevent smooth melting). Measure out the stock, milk, and seasonings, and mince the fresh thyme.

Melt the butter in a medium saucepan over medium heat. Add the diced onions and cook for 2-3 minutes, stirring occasionally, until they soften and become translucent. Add the minced garlic and cook for another 1-2 minutes until fragrant. Sprinkle the flour over the mixture and stir continuously for 1-2 minutes to create a roux base—this removes the raw flour taste and will help thicken your sauce. Pour in the vegetable stock and milk gradually while whisking to avoid lumps, then add the salt, white pepper, black pepper, and fresh thyme. Simmer the sauce for 4-5 minutes, stirring occasionally, until it thickens enough to coat the back of a spoon. Remove from heat, stir in the Dijon mustard, and taste to adjust seasonings.

Arrange half of the sliced potatoes in an even layer on the bottom of your prepared baking dish, slightly overlapping them for a professional appearance. Pour half of the sauce from Step 2 over the potatoes, then sprinkle with half of the parmesan and 0.75 cup of the shredded gruyere. Layer the remaining potatoes on top, pour the remaining sauce over them, and finish with the final 0.5 cup of gruyere on top. I like to reserve the parmesan mostly for the middle layer since it's a stronger flavor and benefits from being surrounded by the milder, creamier gruyere.

Cover the baking dish tightly with aluminum foil and place it in the preheated 400°F oven. Bake for 25-30 minutes until the potatoes are mostly tender when pierced with a fork. Remove the foil and bake for an additional 20-25 minutes until the top is golden brown and the cheese is bubbling around the edges—this second phase allows the gruyere to develop a beautiful crust while the potatoes finish cooking through.

Remove the scalloped potatoes from the oven and let them rest for 10-15 minutes before serving. This resting period allows the sauce to set slightly, making the dish easier to serve in clean portions and ensuring the creamy texture sets properly. The potatoes will continue to cook gently during this time from carryover heat.