Sauté onion for 3–4 minutes until soft, then add garlic and cook for 30 seconds.
Add ground beef/pork and cook for 5–7 minutes, breaking it up until browned.
Stir in the Bolognese sauce and simmer for 5–10 minutes.
If the sauce is too thick, add 1/2 cup water or broth. Season with salt & black pepper.
Make the Béchamel: In a saucepan, melt 2 tbsp butter over medium heat. Stir in 2 tbsp flour and cook for 1 minute, whisking constantly. Gradually add 1,5 cups milk, whisking until thickened (about 5 minutes). Season with salt & black pepper, add a pinch of nutmeg.
Preheat the oven to 180°C.
In a baking dish, spread a thin layer of meat sauce on the bottom.
Add a layer of lasagna sheets.
Spread 1/3 of the meat sauce over the pasta.
Add 1/3 of the béchamel and sprinkle some cheese.
Repeat layers two more times, ending with meat sauce and cheese on top.
Cover the dish with foil and bake for 30 minutes.
Remove foil, sprinkle more cheese, and bake uncovered for another 10–15 minutes until golden and bubbling.
Let the lasagna rest for 10 minutes before slicing.