Heat the olive oil in a large Dutch oven over medium heat. Add the onion, celery, and carrots, then cook for about 6 minutes until the vegetables begin to soften.
Stir in the garlic and mushrooms. Cook for another 4 to 5 minutes until the mushrooms release their moisture and start to brown.
Mix in the tomato paste, thyme, rosemary, oregano, and smoked paprika. Cook for 1 minute, stirring constantly, to toast the spices.
Add the potatoes, lentils, diced tomatoes, vegetable broth, soy sauce, and bay leaves. Stir everything together until well combined.
Bring the stew to a gentle boil, then reduce the heat to low. Cover and simmer for 25 to 30 minutes, stirring occasionally, until the lentils and potatoes are tender.
Add the broccoli florets during the last 8 minutes of cooking so they stay bright and slightly crisp.
Stir in the spinach and cook for another 2 to 3 minutes until wilted.
Remove the bay leaves. Stir in the lemon juice and chopped parsley, then taste and adjust the seasoning with additional salt and pepper if needed.
Ladle the stew into bowls and serve warm with crusty bread or cooked rice if desired.