Layer the potato, zucchini and spring onions in a greased oven proof dish (I used an 8 inch square), seasoning with the salt and pepper and thyme as you go.
Repeat the layers until all the vegetables are used. Beat together the eggs, cream/milk, and Dijon mustard.
Pour evenly over the layers.
Grate some Parmesan over the top, and bake for 30-40 in a preheated 180 degree oven. (You know it’s cooked when it doesn’t jiggle in the middle when shook.)
Notes
If the top is browning too quickly, cover it with foil.