Preheat the oven to 200°C/fan 180°C/gas mark 6. Line a 30cm x 20cm tin with baking paper, making sure it overlaps on the sides. Add the vegetables to the tin, season and toss with the rosemary, thyme and olive oil. Roast for 15 minutes. Add the garlic and roast for an additional 10 minutes. Remove from the oven, remove the herb stalks, and turn the oven down to 180°C/160°C fan/gas mark 4.
Whisk the eggs together with the milk, parmesan, parsley and some seasoning. Scatter the smoked mackerel over the vegetables, then pour over the egg mixture. Bake for 30 minutes or until set and golden brown on top. Leave to cool slightly, then cut into squares and serve with the salad leaves.