Rinse the red lentils under cold running water until clear. Dice the onion, mince the garlic, peel and dice the carrots, dice the celery and bell pepper, drain and rinse the chickpeas, chop the spinach, and measure all spices into a small bowl.
Heat olive oil in a large Dutch oven over medium heat. Add the diced onion and cook for 5–6 minutes until softened and translucent, then add the minced garlic and cook for 1 minute, stirring constantly.
Add the diced carrots, celery, and red bell pepper to the pot. Stir to coat and cook for 6–8 minutes until the vegetables begin to soften and develop light color on the edges.
Reduce heat to medium-low and add the entire spice blend. Stir constantly for 1–2 minutes until the spices are fragrant and coat all the vegetables evenly, being careful not to let them burn.
Stir in the rinsed red lentils, then add the canned diced tomatoes (with juices) and vegetable broth, scraping up any browned bits from the bottom. Bring the mixture to a boil over medium-high heat.
Reduce heat to low, partially cover, and simmer for 20–25 minutes, stirring occasionally, until the lentils are completely tender and beginning to break down into a creamy consistency.
Stir in the drained chickpeas and cook for 3–4 minutes until heated through, then add the chopped spinach and stir until wilted, about 1–2 minutes.
If the soup is too thick, stir in additional broth ½ cup at a time. Taste and adjust seasoning with salt, pepper, or extra spices, then stir in the fresh lemon juice.
Remove the pot from the heat and stir in the chopped cilantro and parsley. Let the soup rest for 5 minutes before serving to allow the flavors to meld.
Ladle into bowls and top with a dollop of yogurt, a drizzle of olive oil, extra fresh herbs, and a spoonful of harissa if desired. Serve with warm pita or crusty bread.