How to Make It
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.
Tenderize hardy greens like kale by blanching them in a large stockpot of boiling water. Cook the kale in salted water until tender, and then drain the kale and pat dry with paper towels. Look for bags of prewashed and chopped kale to save prep time.