Preheat your oven to 400°F and line a baking sheet with parchment paper.
Peel and dice the sweet potato into 1/2-inch cubes.
In a small bowl, toss the sweet potato cubes with 0.5 tsp of olive oil, garlic powder, and a pinch of salt and pepper.
Spread the sweet potatoes on the baking sheet and roast for 20-25 minutes until tender and slightly caramelized.
While the potatoes roast, rinse the beluga lentils under cold water.
In a small saucepan, combine the lentils and water. Bring to a boil, then reduce heat and simmer for 15-20 minutes until tender but still holding their shape.
Drain any excess water from the lentils and stir in a squeeze of lemon juice.
Season the tuna fillet on both sides with the remaining sea salt and black pepper.
Heat the remaining 0.5 tsp of olive oil in a non-stick skillet over medium-high heat.
Sear the tuna for approximately 2 minutes per side for a medium-rare center, or longer if preferred.
To serve, place the lentils and roasted sweet potatoes on a plate, top with the tuna fillet, and garnish with fresh parsley and the remaining lemon juice.