Preheat the oven to 180°C.
Cut the cauliflower into 2cm thick steaks.
Cut the onion and tomatoes into 4 wedges each, the red pepper into chunks.
Arrange the vegetables in a large roasting dish.
Mix the oil, balsamic vinegar and garlic. Season with salt and pepper, then pour this over the vegetables, coating them well with a silicon brush.
Roast in the oven for about 15 minutes. Check the cauliflower with a fork - it should be able to just go through the steak by now. Pour over the stock and bake for another 10 minutes.
Add the peas and lemon juice. Bake for 5 more minutes, until the vegetables are tender.
Add some mint leaves, then serve with the jus.