Bring a large pot of salted water to a boil and cook the fettuccine according to package directions until al dente. Drain the pasta and set it aside, reserving 1/2 cup of the starchy pasta water.
While the pasta cooks, brown 1 pound of ground beef in a large skillet over medium-high heat, breaking it up with a wooden spoon until no pink remains, which should take about 8-10 minutes. Season the beef with 1 teaspoon kosher salt, 1/2 teaspoon black pepper, and 1 teaspoon garlic powder.
Add the finely chopped onion to the skillet with the beef and cook for 4-5 minutes until softened. Add the minced garlic and cook for 1 more minute until fragrant. Sprinkle the 2 tablespoons of flour over the meat mixture and stir to coat, cooking for 1 minute to remove the raw flour taste.
Slowly pour in the 1 1/2 cups of whole milk, 1 cup of beef broth, and 1 tablespoon of Worcestershire sauce, stirring constantly to create a smooth sauce. Bring the mixture to a simmer and let it thicken slightly for 2-3 minutes.
Reduce the heat to low and stir in the 1/2 cup of heavy cream, 1/2 cup of grated Parmesan cheese, and 2 tablespoons of butter until the cheeses are melted and the sauce is creamy and smooth.
Add the drained pasta to the skillet and toss to coat thoroughly in the sauce. If the sauce is too thick, add the reserved pasta water a few tablespoons at a time until it reaches your desired consistency. Fold in the 1 cup of shredded cheddar cheese until just melted. Serve immediately, garnished with optional sesame seeds and with sliced dill pickles on the side.