Cook the onions: In a large skillet, heat the olive oil and 2 tablespoons of butter over medium-low heat. Add the onions and cook slowly—about 25 to 30 minutes—until they’re soft and light golden but not too dark. Stir occasionally so they don’t stick or burn. You can also add a splash of water or broth if necessary. Combine the onions, parsley, thyme, and garlic in a large bowl; add salt and pepper to taste.

Parboil the potatoes: Meanwhile, bring a pot of salted water to a boil. Add the potato slices and cook for 3 minutes once the water is boiling again. Drain and let them cool slightly, then add them to the onion mixture and toss gently to mix. Preheat your oven to 400 F (200 C). Lightly butter a 3-quart baking dish (9-by-13-inch) or spray it with cooking spray. Transfer the mixture to your baking dish, gently pressing it down just a bit to flatten. Dot the top with the remaining tablespoon of butter.

Roast: Bake for 35 to 45 minutes or until the potatoes are tender and the top is golden and crisp.

Finish and serve: Garnish with more fresh parsley and serve. Enjoy!