To make the batter, sift the flour and salt into a mixing bowl, make a well in the centre, add the egg and whisk, gradually adding the milk, until you have a smooth creamy batter. Whisk in the melted butter. Cover and leave to stand for 30 minutes. Meanwhile, cook the pancetta slices in a large non stick frying pan until crispy - because the slices are so thin they wont take long at all. Snip into small pieces.
Divide the broccoli into small florets and cook in a pan of lightly salted boiling water for 3 minutes, drain well and toss with the pancetta, set to one side. To cook the crêpes, you will need a small pan (also known as a blinis pan) about 10 to 12 cm in diameter. Heat the pan and wipe with a little of the oil. Swirl a little of the batter over the base - cook for 1 to 2 minutes until one side is cooked and pale golden then flip over and cook the other side.
Arrange the cooked crêpes onto a baking tray –( you may need 2 trays) Top each with a little of the broccoli and pancetta mixture then sprinkle with the Gruyère cheese. Pop under a medium grill for 2 minutes until the cheese begins to melt, serve at once.