Preheat your oven to 190-200ºC.
Place the halved and deseeded red peppers cut side up on a roasting tray. Sprinkle with the chopped rosemary and thyme leaves. Drizzle with olive oil and toss with the roughly chopped onion and peeled garlic cloves. If using, sprinkle the light brown sugar over the top.
Roast in the preheated oven for about 15 minutes, until the peppers are soft and charred on the edges.
Once roasted, transfer the peppers, onions, and garlic to a medium saucepan. Add the 500 ml vegetable stock.
Using an immersion blender, blend the soup until thick and smooth. Add the basil leaves and blend again until the basil is fully incorporated. Taste and adjust seasoning as necessary, keeping in mind that the stock cube is already salty.
Stir in the lemon juice to brighten the flavors.