Preheat the oven to 180°C, 160°C fan, gas mark 4.
Use a little of the butter to grease a 22cm square baking dish, then use the rest to butter each slice of bread thinly on one side. Spread the tomato and chilli chutney on top, then cut each slice in half diagonally.
Heat the olive oil in a frying pan and cook the onion for 5-6 minutes, until soft. Start to layer up the bread with the red onion and cherry tomatoes, scattering a few of the veggies on top to finish.
Lightly whisk together the eggs and milk in a jug and season well. Reserve a tablespoon of cheese and add the rest to the egg mixture, before pouring all over the bread. Scatter over the reserved cheese and then bake in the oven for 35-40 minutes until crisp and golden on top.
Serve sprinkled with the fresh chives and the salad leaves on the side.