Fill a large mixing bowl halfway with cold water. Peel 3 pounds potatoes and slice them approximately ½ cm thick. Place sliced potatoes into water to prevent browning (optional). Preheat oven to 350 degrees F.
In a medium-sized saucepan, add ¼ cup vegan butter on medium heat. When melted, whisk in 3 tbsp. flour until smooth. Slowly drizzle in 2 cups vegetable broth, whisking constantly to prevent lumps. Bring to a gentle simmer, stirring often. Gradually whisk in 1 cup cream, along with ½ tsp. kosher salt, ¼ tsp. garlic powder, ¼ tsp. onion powder, ½ tsp. dried tarragon, and dash ground thyme.
Grease a 9x13 inch baking pan. Place half of potatoes into pan, along with half sauce. Ensure potatoes are coated in sauce and the potatoes are quite level across the pan. Sprinkle with 1 cup of the vegan cheese shreds, evenly across. Layer remaining half of potatoes across the pan, then drizzle in remaining sauce over potatoes. Use spatula to ensure potatoes are coated in sauce. Cover pan with foil and place into oven for 40 minutes.
Remove pan from oven, remove the foil, and place back into oven for 20 to 30 minutes. Check for tenderness by piercing the potatoes with a fork. Sprinkle remaining 1 cup vegan cheese shreds evenly over top. Place under broiler on High for 2 to 4 minutes to get some light browning on top. Garnish with fresh thyme leaves.