Bring a large pot of water to boil on high heat. Once water is boiled, add in lasagna noodles and follow package instructions to cook them. Once cooked through, drain and rinse noodles with cold water. Set aside.
Bolognese Sauce
Heat a large saucepan on high and add oil. Saute celery, carrot, onion, and garlic for 10 minutes. Then add in ground beef and Italian sausage. Saute for 5 minutes until sausage and beef are brown.
Add red wine to the beef mixture. Reduce for 5 minutes. Then add in crushed tomatoes and tomato paste. Season with salt, pepper, garlic salt, dried basil, dried oregano, and red pepper flake.
Simmer sauce on low for at least 30 minutes, stirring occasionally. Then add in milk and allow sauce to simmer another 20 minutes. Set aside.
Bechamel Sauce
Take a large saucepan and heat it on medium heat. Place butter in the pan and allow it to melt. Place flour into the pan and mix thoroughly with butter. Cook on medium heat for 3-5 minutes until it turns light beige color. Add in milk and season with salt.
Ricotta Mixture
Make ricotta mixture by mixing ricotta cheese, garlic salt, and egg.
Lasagna Assembly
Place a ladle of sauce on the bottom of a large rectangular baking dish. Then place 3 lasagna noodles on top of sauce, without letting them overlap. Scoop 1/3 of the ricotta mixture on top of noodles and top with a thin layer of bechamel sauce. Add another layer of noodles. Then add a layer of meat sauce, mozzarella cheese, and Parmigiano Reggiano. Top it with another layer of noodles. Repeat until you reach the top of the baking dish. Top the last layer of noodles with meat sauce, mozzarella cheese, and Parmigiano Reggiano.
Cover lasagna loosely with foil. Place in the oven in the middle rack and bake for 45 minutes-1 hr.
Then take off the foil and allow to bake for another 5-10 minutes until the cheese on top becomes bubbly and golden.
Allow lasagna to rest for at least 45 minutes before cutting in. This will allow the layers to hold their shape when you cut into the lasagna.