Heat the oven to 190°C (Gas Mark 5 / 375°F).

Peel the white and red onions, and thinly slice. Add them to a large bowl, along with the unpeeled garlic cloves, olive oil, balsamic vinegar, crumbled veggie stock cube, and plenty of black pepper. Toss to coat. Spread the onions out onto two large baking trays. Roast for around 25-30 minutes, stirring halfway.

When the onions are soft and just beginning to char around the edges, transfer them to a large saucepan, popping the garlic cloves out of the skin. Add the hot water, sugar, Marmite and whole sprigs of thyme, and bring to a simmer. Cook for around 30 minutes, or until ready to serve.

To serve, remove the stalks of thyme (don't worry if the leaves have detached), and ladle the soup into bowls. Top each bowl with a slice of French bread, and add the grated gruyere. Put the bowls under the grill (broiler) for a minute or two, to melt the cheese. Serve immediately.