Preheat oven to 200°C.
Wash green beans and cut them into 3cm pieces. Chop onions finely and cut bacon into small cubes. Boil green beans in salted water until firm to the bit, strain and refresh them with cold water.
Heat oil in a pan and roast onions and bacon. Mix well flour, eggs, milk, cheese, salt and pepper. Cover a gratin dish with butter, pour in half of the mixture and place into the oven for about 30 minutes. Then cover with green beans and at last with bacon-onion mixture. Pour over the second half of the mixture and bake all in oven 30 minutes more.