While the potatoes are cooking, cook the onions in a little oil until transparent, add the minced beef and brown. Add the tomatoes, tomato purée and seasoning. Cook for several minutes.
Prepare a sauce with the butter, flour and milk, flavouring with the bay leaf, and finally add the cheese.
When the potatoes are soft, drain well and make a purée.
Put half the meat in the bottom of a casserole dish, then half the tomato sauce, half the cheese sauce and then repeat. Cover with the potato purée (dot with more butter if required) and put in a hot oven