Place white potato slices on a baking sheet and drizzle olive oil over each; season with garlic, thyme, salt, and pepper.
Roast in the preheated oven until tender, about 30 minutes.
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels and crumble.
Place russet potatoes into a large pot and cover with chicken broth; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain.
Mash boiled russet potatoes with butter in a bowl until evenly mixed; mix in crumbled bacon, Cheddar cheese, and milk. Transfer mashed potato mixture to a piping bag with a large star tip or plastic bag with a corner snipped. Pipe mashed potato mixture onto each roasted potato slice and top with chives.