In a large stock pot, heat the oil and saute the garlic, onions, celery and carrot until the vegetables are tender.
Add the rinsed lentils, water and beef stock.
Bring the mixture to a boil, then reduce the heat to a simmer, cover and cook for about 45 minutes (or until the lentils are tender), stirring occasionally.
Add the tomatoes (with the juice in the can) and the chopped oregano, mint (if using), salt and pepper.
Stir in the spinach and simmer uncovered for about 15-20 more minutes.