Cheesy Cauliflower Gratin with Bacon Bread Crumbs

Preheat oven to 375 degrees F. Butter a 9-x-13" baking dish. Bring a large pot of salted water to a boil. Boil cauliflower for 5 minutes, then drain. In a saucepan, melt 2 tablespoons butter over medium-high heat. Add flour, and stir constantly until lightly browned, 4 minutes. Stir in warm milk, whisking constantly, and bring to a boil. Reduce heat to low and simmer until thick, 8 minutes. Whisk in 1/2 cup Gruyere and 1/2 cup cheddar. Season with salt and pepper and nutmeg. Meanwhile, in a large skillet over medium heat, cook bacon until crispy, 6 to 8 minutes. Transfer to a paper towel-lined plate to cool, then crumble and add to a small bowl with panko bread crumbs, Parmesan, and thyme. Season with salt and pepper. Arrange cauliflower in buttered baking dish and pour over cheese sauce. Sprinkle with remaining 1/4 cup Gruyere and 1/4 cup cheddar and top with bread crumb mixture. Drizzle with olive oil. Bake until cheese is bubbling, 20 minutes. Increase heat to broil and broil until bread crumbs are deeply golden,1 minute. Let cool slightly, then serve.