Immerse the conchiglioni in a large pan of boiling salted water and cook according to the instructions on the packet for "al dente", reducing the recommended time by 2 minutes.
Refresh in cold water, drain and place in a mixing bowl with a dash of olive oil.
Gently fry the peeled and finely chopped shallots with the olive oil and then add half of the tomato sauce along with the thyme and the flaked tuna.
Season and leave to simmer for 5 minutes then carefully stuff the pasta shells using a teaspoon.
Pour the rest of the sauce into a large, greased baking dish and arrange the pasta, wedged closely together.
Add a slither of mozzarella to each stuffed shell, sprinkle with parmesan, and cook in an oven preheated to 180°C (160° fan) | 350F | gas 4 for 15 to 20 minutes.