Preheat oven to 350°. Spray a 13x9-inch baking dish with nonstick cooking spray. Set aside.
In a large Dutch oven, cook bacon over medium heat until crisp. Remove from pan using a slotted spoon. Let cool; crumble. Reserve 3 tablespoons bacon fat in pan.
Add flour and garlic to pan. Cook, whisking constantly, 1 minute. Gradually whisk in milk, 2 teaspoons salt, and pepper. Cook, whisking frequently, until thickened, approximately 5 minutes.
Stir in tomatoes with green chiles, 1 1/2 cups Cheddar, and Parmesan until melted. Remove from heat. Stir in cooked pasta, chicken, remaining 1 teaspoon salt until combined. Pour mixture into prepared dish
In a small skillet, add panko and butter. Cook over medium heat until toasted, about 4 minutes. Sprinkle panko over casserole; top with remaining ½ cup cheese and bacon.
Bake until cheese is melted and lightly browned, 15 to 20 minutes. Let stand 5 minutes before serving. Garnish with green onion, if desired.