Preheat broiler with oven rack 8 to 9 inches from heat. Bring 4 quarts water and 1/4 cup of the salt to a boil in a Dutch oven over high. Add chopped kale; cook until tender, 5 to 7 minutes. Drain and pat dry with paper towels. Cool 10 minutes.

Meanwhile, place milk in a 1-quart glass measuring cup. Cover with plastic wrap, and microwave on HIGH 3 minutes.

Melt butter in Dutch oven over medium. Add onion, and cook, stirring occasionally, until tender, about 6 minutes. Add flour, and cook, whisking constantly, 2 minutes. Whisk in hot milk. Increase heat to medium-high, and bring to a low boil, whisking often. Cook, whisking often, until thickened, about 6 minutes. Remove from heat; whisk in cheese, hot sauce, pepper, and remaining 1 1/2 teaspoons salt. Roughly chop kale. Fold kale and cooked pasta into cheese sauce. Pour into a greased 11- x 7-inch baking dish.

Stir together panko and 1 tablespoon olive oil; sprinkle over pasta mixture. Broil until panko is golden brown, 1 to 2 minutes. Serve immediately.