Preheat the oven to 180°C.
Cut the tomatoes in half, and the red peppers into 4 large pieces. Arrange them in a large baking tray.
Cut both onions into 4 wedges and tuck them between the tomatoes together with the garlic cloves (don’t peel them).
Add the herbs, balsamic vinegar, oil and chilli flakes. Season with salt and pepper. Toss well.
Put the tray in the oven and bake for approx. 30 minutes until the vegetables are soft and a little charred.
Take the tray out of the oven and let it cool.
Remove the skins of the onion, garlic and peppers (tomatoes too, if you want a really silky soup).
Put all the roasted veggies and the cooking jus into a saucepan. ( If you want to make the soup later, put them in a jar and store in the fridge up to 3 days).
Add the stock and spinach to the pan. Turn the heat on and bring to a simmer. Turn off the heat and, using a stick blender, blitz the mixture into a smooth soup. Add more stock if it’s too thick.
Drain the lentils and add to the soup. Bring to a simmer to warm through.
Serve with a squeeze of lemon juice and plenty of freshly ground pepper.