Preheat the oven to 450 degrees F. Grease 2 sheet pans with a thin and even coating of olive oil.
Slice the potatoes into 1-inch-thick rounds.
Place the potatoes and 1 tablespoon kosher salt in a large pot. Fill it with cold water, covering the potatoes by 1 inch. On the stove over high heat, bring the water to a boil, reduce the heat and simmer the potatoes until they are just tender enough to be pierced with a knife, about 5-7 minutes.
Drain the potatoes and place them on the pans, spacing them about 1-2 inches apart. Use a potato masher, the bottom of a glass or a fork to smash them down.
In a small bowl, stir 1/4 cup olive oil, thyme, garlic powder, onion powder, 1/2 teaspoon kosher salt and black pepper. Drizzle the tops of the potatoes with this oil mixture.
Roast the potatoes for 30-32 minutes, rotating the sheet pans halfway through roasting, until the bottoms and edges are browned and crispy.
Sprinkle the potatoes with walnuts and chives before serving.