In a large saucepan, heat olive oil over medium high heat. Add sausage and cook for 10 minutes, browning on all sides. Remove and set aside.
Add garlic, celery, carrots and onions to saucepan and cook 5 to 7 minutes or until vegetables begin to get tender. Stir in cumin, thyme and lentils and cook 1 minute more.
Add stock and bring to a boil. Reduce to a simmer and cook for 30 to 35 minutes or until lentils are tender but not mushy.
Stir in spinach and reserved sausage and cook 5 minutes to heat through.