Heat Oil: Heat the oil in a large soup pot over medium heat. If you're not using oil, just add the onion with 1-2 tbsp water or broth.
Saute Onion: Add the onion and a pinch of salt and cook for 4-5 minutes, stirring often.
Add Fresh Veggies: Add the garlic, celery and carrot and cook for another 3-4 minutes until softened, stirring often.
Add Seasonings: Add the pepper, dried herbs, red pepper flakes, if using, and the bay leaf. Cook for 1 minute while stirring.
Add Liquids and Simmer: Add 5 cups of the stock, red lentils and diced tomatoes and bring to a light simmer. Start with 5 cups of broth and if needed, slowly add a bit more broth or up to a cup of water as it cooks to adjust the thickness. Simmer lightly, uncovered, until the lentils are tender, about 15-20 minutes.
Season and Serve: Season with salt and additional pepper, to taste, and add a squeeze of lemon juice. Remove the bay leaves and serve right away.