Preheat oven to 400°F. Place a steamer basket into a large pot and fill with a few inches of water. Heat to a boil on high. Meanwhile, microwave the bacon according to package directions and reserve.
Peel the potatoes and cut into ½-inch chunks. When water is boiling, add potatoes to the steamer basket. Cover and steam 5 min. Add the Brussels sprouts and steam with potatoes 8–10 min., until all vegetables are tender. Remove vegetables from steamer basket and transfer to a medium bowl.
Coat a 1½-quart baking dish with cooking spray. Chop cooked bacon. To bowl with vegetables, add bacon, half & half, 1 cup Gruyère, Parmesan, and thyme. Season with salt and pepper. Stir to combine. Transfer to baking dish.
In a small bowl, combine the bread crumbs, oil, and remaining ½ cup Gruyère cheese. Sprinkle over mixture. Bake 10 min. Reset oven to broil on high. Broil 3–5 min., until top is golden brown.