Preheat the oven to 180C/160C fan. Grease a baking dish with butter.
Fry the onion and red pepper in a little oil for 5 minutes until softened. Set aside to cool slightly.
Tear the bread into chunks and spread them in the baking dish. Scatter over the cooked onion and pepper and the frozen peas.
Whisk together the eggs, milk, mustard, dried herbs and half the cheese. Season well with salt and pepper.
Pour the egg mixture evenly over the bread. Press down gently so the bread soaks up the liquid. Leave for 5 minutes.
Scatter the remaining cheese on top. Bake for 30-35 minutes until golden, puffed up and set in the middle.
Let it rest for 5 minutes before serving. It will deflate slightly - that is normal.