Spread the butter thickly over the base and halfway up the sides of a medium-sized frying pan with an oven-proof handle. Shake the sugar evenly over the butter and sprinkle with lemon juice. Peel and cut all but 2 of the bananas into 3 cm (1 inch) lengths and stack them upright over the base of the pan, packing them in closely. Cut the remaining bananas in half vertically to go around the edges.
Remove the pan from the oven and leave for 10 minutes, work round the edge of the tart with a knife, then carefully invert it onto a large plate. If you find a few pieces of banana have stuck to the base of the pan, simply pry them off with a round-bladed knife and stick them back on top of the tart, caramelized-side upwards.