I much prefer sugar snap peas to mange tout, but either can be used.
Pinch off the ends of the peas and put the peas on one side.
Melt the butter with the olive oil in a deep sauté pan over a medium heat. Add the prepared leeks and the pumpkin seeds and stir often until the leeks are soft but not browned. Season with salt and plenty of black pepper. Meanwhile, bring a saucepan of salted water to the boil, add the prepared peas and boil for a few minutes