Heat the oven to 190 C / 375 F. Put the rinsed lentils and 360 ml water in a 2-quart / 1.9 L saucepan, bring to a boil, then simmer for 18 to 20 minutes until tender but still holding their shape. Drain in a fine-mesh sieve and shake for a full minute so no water is left.
While the lentils simmer, heat the olive oil in a 30 cm / 12-inch skillet over medium-high heat. Cook the onion for 4 minutes until soft. Add the turkey and break it up, cooking 7 to 8 minutes until no pink remains and it's browned in spots.
Stir in the garlic, chili powder, cumin, smoked paprika, salt and pepper and cook for 1 minute until fragrant. Add 240 g (1 cup) of the tomato sauce and simmer for 4 to 5 minutes until a spoon leaves a trail through the filling.
Fold the drained lentils into the turkey mixture and take the skillet off the heat. Spread the remaining tomato sauce (about 185 g) across the bottom of a 23 x 33 cm / 9 x 13 inch baking dish. Set the pepper halves cut-side up on the sauce and mound about 110 g of filling into each half.
Cover the dish with foil and bake for 25 minutes. Remove the foil, scatter the cheddar over the peppers, and bake 10 more minutes until the cheese is melted and spotted brown, a paring knife slides into the pepper with no push, and the filling reads 165 F / 74 C in the center.
Rest the peppers in the dish for 5 minutes so the filling firms up. Top with sliced green onions and serve one whole pepper (two halves) per person.