Preheat the oven to 180 degrees C.
Melt the butter in a large saucepan over a medium heat. Peel and dice the onion: add to the saucepan.
Stir in the flour. Cook for 2 minutes, then turn off the heat. Gradually add the milk, whisking all the time.
When you have added all of the milk, turn the heat back on and add the bay leaf. Bring back to a simmer and stir until thickened, then add the mustard powder, meat, and some salt and pepper.
Layer lasagne sheets, creamy sauce, then peas. Repeat. Finish with a layer of lasagne sheets then creamy sauce. Top with the grated Parmesan.
Put in the oven and bake for 45 minutes until golden and bubbling.