Prep Kale: Clean the kale by rinsing it in cold water in a colander, shaking off excess water, and then removing the leaves from the stems. Chop the leaves into small pieces.
Saute Onions and Garlic: Heat olive oil on medium in a cast iron skillet or another large skillet (with a lid). Add onion and garlic and cook for approximately 90 seconds.
Cook Kale: Add kale and coat with oil, and mix with garlic and onion. Let it saute for another minute, add 1 teaspoon of salt, mix, and then add the stock. Cover and let braise in stock for about 10 minutes, or until most of the stock has cooked off. Once done, remove the kale from the skillet and set aside.
Cook Pasta: Meanwhile, bring 3 quarts of water to a boil. Add the pasta and boil until al dente. Give a quick rinse with cold water to prevent sticking and then drain.
Create Roux: With the skillet on medium heat, add the butter and as it melts, add the flour and whisk to create a roux. Slowly add the milk, 1 cup at a time, and whisk it to incorporate it all before adding more milk and half and half.
Add Cheese: Let the sauce cook, whisking constantly to prevent burning, until it begins to thicken. Reduce the heat to medium-low. Add the cheese, a handful at a time, whisking until it is all fully melted before adding more cheese. Repeat for 2 cups of cheese (leave the last 1/2 cup for final broil, if not broiling, then add all of the cheese).
Season: Once cheese sauce is thickened, add 1/2 teaspoon salt, pepper, and mustard and mix. Add kale and mix well.
Add Kale and Pasta: Add the cooked kale and mix well. This will ensure that the kale is more evenly distributed in the dish. Add pasta and mix well with the cheese and kale.
NOTE: This is where you can stop and set aside if preparing in advance. Or you can serve it now, as is.
Preheat Oven: Pre-heat oven to broil and set rack to top third.
Broil: Add the last 1/2 to 1 cup of cheese to the top of the skillet and add it to oven to melt and crisp up the cheese for 3-5 minutes (be sure to check after 2 minutes to ensure it's not burning)
Serve and enjoy!