Warm the olive oil in a large pot over medium heat then sauté the onion for several minutes.
Toss in the spinach and the garlic and sauté until spinach is wilted.
Add the vegetable broth, diced tomatoes with liquid, and lentils to the pot along with dried herbs.
Stir to combine and bring heat to a boil.
Once the liquid starts to boil, reduce heat to a simmer. Wait a few minutes then cover with a lid and allow to cook for 30 minutes, until lentils are just tender but not mushy. Remove from heat and squeeze in lemon juice then stir together.