Two yellow onions, peeled and ends trimmed
5 tablespoons olive oil, plus more for greasing
3 cloves garlic, minced
2 teaspoons chopped fresh thyme
4 russet potatoes (about 3-1/2 pounds total weight), ends trimmed
2 teaspoons kosher salt, plus more as needed
1/2 teaspoon freshly ground pepper
6 ounces Gruyere cheese, shredded
Roughly chopped fresh flat-leaf parsley, for serving (optional)
Cut a slit in one side of each onion, stopping near the center. Spiralize the onions using the straight blade.
In a large sauté pan over medium-high heat, warm 3 tablespoons of the oil. Add the onion and sauté until browned and tender, about 15 minutes. Add the garlic and thyme and sauté until fragrant, about one minute. Transfer to a large bowl and let cool
Preheat the oven to 400°F and lightly grease a 4-quart baking dish
Spiralize the potatoes using the shredder blade, stopping to break or cut the strands every 3 to 4 rotations. Transfer the potatoes to the bowl with the onions and toss with remaining 2 tablespoons olive oil, the salt, pepper, and two thirds of the Gruyere until well mixed. Transfer the mixture to the prepared dish and sprinkle the remaining Gruyere over the top. Cover the dish with foil and bake for 30 minutes. Remove the foil and continue to bake until the top is golden brown and the knife inserts easily into the center of the gratin, 20 to 30 minutes.
Let rest for five minutes before serving. Sprinkle with parsley, if desired, and serve.