Start by sautéing together the carrots, onion, and garlic in olive oil. Once the vegetables are browned, transfer them to the slow cooker. You can skip this step if time doesn't allow and place the diced vegetables in the slow cooker without sautéing them. You still will get a delicious result.
Next put lentils, diced tomatoes, drained great northern beans, basil, oregano, salt, and chicken broth. Mix altogether and place the lid on the slow cooker. Cook for 4-6 hours on high.
When ready to serve stir in chopped spinach and mix until well mixed and spinach is welted.
Top each individual bowl with cooked orzo pasta and parmesan for a hearty lentil soup.
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Notes
If you are in a hurry, you can skip sautéing the onion, carrots, and garlic and still have a delicious result. I do prefer the flavor that comes out when you do have the time to saute the vegetables, before putting them in the slow cooker. If you are omitting the sautéing of the vegetables, you can omit the olive oil as well.