Preheat oven to 375 degrees.
In a large pot, bring 6-8 quarts of water to a boil. Add 3 sodium-free chicken boullion cubes to the water along with elbow noodles. Cook for 8 minutes. Drain but DO NOT RINSE and return the noodles to the pot.
Meanwhile, in a large saucepan, melt 4 Tablespoons of butter over medium heat. Add diced onion and sauté until translucent and soft. Add minced garlic and sauté until fragrant.
Next, add flour and cook for about 2 minutes, stirring constantly and taking care not to burn the roux.
Slowly begin to whisk in milk, stirring constantly to eliminate any flour lumps. Allow the milk mixture to cook gently for about 5 minutes, stirring frequently so that milk does not scald.
Once the mixture begins to thicken, add seasoned salt, pepper and dry mustard. Mix well.
Next, add cheese, one handful at a time, whisking well between each addition. Once all of the cheese has been incorporated and is smooth, remove from heat. Gently pour cheese sauce over noodles and mix thoroughly.
Pour noodle mixture into a 13 x 9 casserole dish or a cast-iron skillet. Sprinkle breadcrumbs over top of macaroni and cheese. Place into oven for 35 to 40 minutes or until bubbly and golden brown on top.
Cool on a wire rack for about 10 minutes before serving.