In a saucepan, brown the onion and garlic in a little olive oil for 3 minutes. Add the tomato coulis, thyme, bay leaf, salt, pepper and a tablespoon of olive oil. Cover and simmer over low heat for 30 minutes.
Fry the chard in a little olive oil until the green has faded and the ribs are tender, this takes about 15 minutes.
Prepare the béchamel. Melt the butter, the flour and add the cold milk little by little until it thickens. Add the nutmeg. Taste and adjust the seasoning if needed.
Assemble the lasagna by alternating layers, sprinkle with cheese and bake for 45 minutes in a preheated oven at 190°.
Serve hot.