Peel and finely chop the red onion. Halve the peppers. Scoop out the seeds and white pith, then finely chop the peppers. Warm a large pan over a low heat for 1 min. Add 1 tbsp olive oil, the onion and peppers. Season with salt and pepper. Cover and sweat for 5 mins.
Peel and crush the garlic. Rinse the red lentils under cold running water. Stir the garlic into the veg. Cook and stir for 1 min till the pan smells sweet and aromatic. Squeeze in the lemon juice, then add the lentils and chopped tomatoes.
Add 600ml boiling water to the pan. Cover, turn up the heat and bring to the boil. Turn the heat down a little and simmer for 20 mins. Taste and add more salt and pepper if you think it needs it. Ladle into warm bowls. Garnish with chopped parsley leaves to serve.