Cook the Beef
In a large soup pot or Dutch oven, heat the olive oil over medium heat. Add the beef and cook until browned on all sides, about 5-7 minutes. If using ground beef, cook until browned and crumbled. Remove the beef from the pot and set it aside.
Sauté the Vegetables
In the same pot, add the chopped onion, carrots, and celery. Sauté for 5-7 minutes, or until the vegetables are softened. Add the garlic and cook for an additional 1 minute until fragrant.
Add the Lentils and Broth
Stir in the lentils, beef broth, diced tomatoes (with juice), cumin, paprika, thyme, bay leaf, salt, and pepper. Bring the soup to a boil.
Simmer the Soup
Once the soup is boiling, reduce the heat to low and cover the pot. Let it simmer for 40-45 minutes, or until the lentils are tender and the beef is cooked through.
Finish the Soup
Remove the bay leaf and stir in the lemon juice (if using) for a touch of brightness. Taste the soup and adjust the seasoning as needed with more salt and pepper.
Serve
Ladle the soup into bowls and serve hot. You can garnish with fresh herbs like parsley or cilantro if desired.