Preheat the oven to 180°C / 350°F.
Peel and dice the onion.
Clean the pepper and cut it into small cubes.
Drain the olives.
Warm the milk and add the cream cheese. Whisk until smooth and fully combined. It should be creamy and easy to pour over the pasta.
Heat a pan and pour in the oil from one can of tuna. Add the onion and pepper, and sauté them in the tuna oil.
Reduce the heat and add the tomato purée. (If it splashes, remove the pan from the heat for a moment.) Add the tuna with its liquid, then the olives, and season to taste. Be sure to taste it, as the olives also add saltiness. Simmer everything together.
Prepare a baking dish and pour in one-third of the tomato sauce. Place three lasagna sheets on top, then spread cream cheese over them.
Add another layer of tomato sauce, followed by three more lasagna sheets, and again spread cream cheese on top.
Pour over the remaining sauce, then place the last three lasagna sheets on top. Spread the remaining cream cheese over them and finally cover with grated cheese.
Cover with aluminum foil and bake in a preheated oven at 180°C / 350°F for 40 minutes.
After 30 minutes, remove the foil and bake for another 10 minutes, until the cheese becomes golden and slightly crispy on top.