Lentils and chicken together in one bowl means double the protein at lunch. This soup is warming, filling, and gets better with each reheat.
Preheat a grill pan or outdoor grill to medium-high heat. Pat chicken breasts dry, sprinkle both sides with 0.5 tsp salt and 0.25 tsp black pepper.
Grill the chicken for 5–6 minutes per side until just cooked through (internal temp 165°F). Transfer to a plate, tent with foil, and let rest.
While chicken cooks, heat 2 tbsp olive oil in a large soup pot over medium heat. Add diced onion, carrot, and celery. Sauté for 6–7 minutes until softened.
Stir in minced garlic, smoked paprika, and cumin. Cook for 1 minute until fragrant, stirring constantly to prevent burning.
Add lentils, chicken broth, and remaining salt and pepper. Bring to a boil, reduce heat to a gentle simmer, and cook uncovered for 25–30 minutes until lentils are tender but not mushy.
Dice the rested chicken into bite-sized pieces. Stir chicken and optional baby spinach into the soup and simmer 2–3 minutes until spinach wilts.
Turn off the heat and stir in lemon juice. Taste and adjust seasoning. Serve hot, garnished with fresh herbs if desired.