Preheat oven to 180°C (360°F). Grease a 19cm square (or similar) baking dish or tin and line with grease-proof paper, allowing a good couple of inches of over-hang (to allow you to lift the slice out after it’s cooked).
If using bacon, slice off the rind and pan-fry until lightly crisp. Drain and put aside.
Grate unpeeled zucchinis and carrot – gently squeeze out any excess liquid using a kitchen paper towel or tea-towel. Chop onion finely. If using ham, slice finely.
In a small bowl, combine the eggs, mustard and oil and whisk until combined.
Put all the ingredients – the vegetables, ham (or bacon) and egg mixture – into a large bowl, along with the flour and cheese. Season with salt and pepper.
Pour the batter into the baking dish or tin and bake for 45-50 mins (mine took closer to an hour). It should be golden on the top and firm to the touch. A skewer inserted should come out dry.
Serve with your favourite relish and a fresh salad. Store any left-overs in the fridge for the next day, or cut into slices, wrap in plastic wrap and freeze. It can be re-heated in the oven or the microwave.