Heat olive oil, onion, and garlic in a large soup pot over medium heat until onion softens, about 2-3 minutes.
Add remaining ingredients, except for the spinach, and bring to a boil over high heat. Reduce heat to low and simmer uncovered, stirring occasionally, until the lentils are becoming thick, 15 to 20 minutes.
Add chopped spinach and stir it in until it wilts, about 1-2 minutes. Season to taste and serve.
Notes
*Or 1 10-ounce package frozen chopped spinach.Make Ahead: Can be made up to a day ahead. If making ahead, wait until reheating to add the fresh spinach.