Adjust an oven rack 8-9 inches from the heat, then turn the oven on the broil setting.
Grease a 7x11-inch baking dish.
In a large Dutch oven over high heat, add the water and 1/4 cup of the salt and bring to a boil.
Add the chopped kale to the boiling water and cook until it is tender, about 5-7 minutes.
Drain the cooked kale and pat it dry with paper towels. Wipe the Dutch oven clean.
Let the kale cool, about 10 minutes.
In a 1-quart glass measuring cup, add the milk, cover with plastic wrap, and microwave until it is warm, about 3 minutes.
In the same Dutch oven over medium heat, melt the butter.
Add the onions to the melted butter and cook, stirring occasionally, until they are tender, about 6 minutes.
Add the flour to the onions and cook, whisking constantly, about 2 minutes.
Add the warm milk to the onion-flour mixture and whisk to combine.
Increase the heat to medium-high. Bring the milk mixture to a low boil and cook, whisking frequently, until it has thickened, about 6 minutes.
Transfer the Dutch oven from the heat, then add the Monterey Jack cheese, the hot sauce, the black pepper, and the remaining salt to the milk mixture. Whisk to combine.
Roughly chop the cooled kale.
Add the chopped kale and the cooked pasta to the cheese sauce and stir to combine.
Pour the pasta mixture into the prepared baking dish.
In a medium bowl, add the panko breadcrumbs and the olive oil and stir to combine. Sprinkle it over the pasta mixture.
Broil the pasta bake until the panko breadcrumbs are golden-brown and the bake is heated through, about 1-2 minutes.
Serve.