Cook lentils: In a saucepan, combine lentils and broth. Bring to a boil, reduce heat, and simmer 18–20 minutes until tender. Drain any excess liquid.
Make dressing: Whisk olive oil, lemon juice, mustard, garlic powder, salt, and pepper in a bowl until emulsified.
Combine veggies: In a large mixing bowl, toss cooked lentils with cucumber, tomatoes, onion, and herbs.
Add avocado: Gently fold in diced avocado to avoid mashing.
Dress salad: Pour lemon dressing over mixture and toss lightly to coat.
Season & serve: Adjust seasoning, sprinkle red pepper flakes if using, and divide into bowls.
