Heat 1 tablespoon of olive oil in a large saucepan over medium heat, and cook and stir the carrots, red onion, garlic, and salt and pepper until the onions are translucent, about 5 minutes. Stir in the lentils and water, bring to a boil, and reduce heat to a simmer. Cook the lentils until tender but not mushy, about 20 minutes.
Drain off excess liquid, and transfer to a bowl. Stir in 3 tablespoons of olive oil, 1 1/2 tablespoons of lemon juice, parsley, and additional salt and black pepper if needed.
Drizzle 1 1/2 tablespoons of lemon juice over the tuna steaks, and sprinkle both sides of the steaks with salt and pepper. In a large skillet, heat 2 tablespoons of olive oil over medium-high heat until the oil is very hot, place the steaks into the hot oil, and sear on each side until the outside of the tuna is well browned and the inside is pink, about 2 minutes per side. Divide the lentil salad between 4 plates, and serve a tuna steak next to the salad.
