Heat a tablespoon of olive oil in a large heavy-based pot on MED-HIGH. Add the ground pork and beef. Break the meat up with a spoon. Cook for 10–15 minutes, until it is browned.
Remove the meat from the pot. Drizzle in the rest of the olive oil. Add in the chopped onions, celery, carrots, and garlic. Sauté for 5–8 minutes.
Return the meat to the pot along with the tomato paste, crushed tomato, beef stock, Italian herbs, and a pinch of salt and pepper. Stir it all together. Cover it with the lid, and bring to a boil. Once the pot is boiling, turn down the heat and let it simmer for 45 minutes.
The Cheese Sauce:
When there's 15 minutes left for the meat, preheat the oven to 350°F. In a non-stick skillet or a saucepan, melt the butter. Whisk in the flour until the mix becomes slurry smooth. Reduce the heat to LOW. Slowly pour in the milk, whisking as you go until the sauce thickens.
Add the shredded Gruyere, Parmesan, and cheddar and whisk. Remove the sauce from the heat and set it aside.
The Lasagna:
In a large casserole dish, spoon in a layer of the meat sauce. Cover completely in a single layer of lasagna sheets. Spoon on a layer of cheese sauce. Repeat this to make 3 layers or all ingredients have been used.
Sprinkle on the cheddar cheese. Place the lasagna in the oven and bake for 45 minutes.