Combine the water and 1 teaspoon salt in large saucepan over high heat. Add lentils and bring to boil. Cover, reduce heat to medium, and simmer until lentils are tender, about 20 minutes. Drain and rinse with cold water, drain again. Place in large bowl.
Dressing
In a blender or using a immersion hand blender, mix the olive oil, vinegar, mustard, 1 tablespoon of berbere, cinnamon, garlic, and 1 teaspoon salt, until thick and emulsified. Pour 3/4 cup dressing over warm lentils and toss to coat, set aside.
Chicken
Rub chicken all over with remaining 1 tablespoon berbere and 1 teaspoon of salt.
Heat 2 tablespoons olive oil in large skillet over medium heat. Add onion and sauté until soft and caramelized, about 8 minutes. Remove and set aside. Add chicken breasts to skillet and sauté 3 minutes per side, or until cooked through.
Serving
Toss the lentils with the dressing again. Arrange lentils on large platter or individual plate. Place chicken on top of lentils, onions on top of chicken and drizzle with remaining dressing and sprinkle with parsley. Serve with lemon slices.