Cook the vegetables: Heat 2 tablespoons of the vegetable broth in a large pot or Dutch oven over medium heat. Add the celery, carrots, yellow squash, and zucchini and cook the vegetables over medium heat, stirring occasionally, for approximately 8 minutes.
Season the vegetables: Add Italian seasoning, onion powder, and garlic powder to the pot and continue to cook over medium heat, stirring occasionally, for an additional 1 to 2 minutes, until the vegetables are coated with seasoning and have softened. If needed, add 1 to 2 tablespoons of vegetable broth to prevent sticking.
Add the lentils, liquids, and tomatoes: To the pot, add the rest of the vegetable broth, water, diced tomatoes with their juices, red lentils, and tomato paste. Bring to a boil, then reduce the heat to low and simmer, uncovered, for 30 to 35 minutes, or until the vegetables are fork-tender.
Add the finishing touches: Stir in the spinach leaves, nutritional yeast, and 1 teaspoon of lemon juice. Add cracked black pepper and additional lemon juice, a half teaspoon at a time, to taste. Remove from heat and serve when the spinach leaves have wilted.
If including pasta (optional): While the soup is simmering, in a separate pot, cook the pasta according to the package directions. Drain and stir the cooked pasta into the soup at the end.