Garlic / Onions / Carrots / Celery - Prep as directed. (Can be done up to 4 days ahead)
Lentils - Rinse.
Tomatoes - Drain.
If slow cooking - Combine garlic, onions, carrots, celery, lentils, tomatoes, bay leaves, Italian seasoning, salt, black pepper, and broth in slow cooker (note: while you could cook barley with the rest of the ingredients, the result is much slimier, so we highly recommend cooking separately). Cook on low for 7 to 8 hours or high for 3.5 to 4 hours. (Can be done up to 5 days ahead)
Barley - Combine barley, water, and some salt in a saucepan, using a 1:3 barley to water ratio. Bring to a boil and then simmer covered until water has been mostly absorbed and barley is tender and chewy (~25 to 30 minutes for pearled barley or ~40 to 50 minutes for hulled barley). (Can be done up to 5 days ahead)
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If not slow cooking - Heat a Dutch oven or large pot over medium-high heat. Add some oil and then garlic, onions, carrots, and celery to heated oil. Saute for 5 to 7 minutes and then add lentils, tomatoes, bay leaves, Italian seasoning, salt, black pepper and broth. Bring to a boil and then simmer for 30 to 40 minutes until lentils and veggies are tender. Turn off heat.
Remove bay leaf and stir in barley and spinach and season to taste with more salt, pepper, and spices. Divide into bowls and serve with a dollop of sour cream.