Warm the olive oil in a large pot over medium heat until it shimmers and begins to gently sizzle.
Add the chopped onion, carrots, and celery to the pot. Stir occasionally and cook until the vegetables soften and release a fragrant aroma, about 5 minutes.
Mix in the minced garlic, turmeric, and cumin. Cook for another minute until the spices are fragrant and slightly toasted, filling the kitchen with warm, earthy scents.
Pour in the vegetable broth and add the rinsed lentils. Bring the mixture to a boil, then reduce the heat to low and cover the pot.
Let the soup simmer gently for about 25 minutes, stirring occasionally, until the lentils are tender and beginning to break down.
Stir in the chopped zucchini and spinach, if using. Continue simmering for another 10 minutes until the vegetables are cooked through and the soup thickens slightly.
Remove the pot from heat. Squeeze fresh lemon juice into the soup and stir well to brighten the flavors.
If desired, use an immersion blender to puree a portion of the soup directly in the pot for a creamier texture, or leave it chunky for more bite.
Taste and season with salt and pepper as needed. Let the soup sit off the heat for a few minutes to allow flavors to meld.
Ladle the hot soup into bowls, garnish with fresh herbs if desired, and enjoy the warm, comforting flavors.