In a large sauce pot over medium-high, heat the oil until shimmering. Add the onion, garlic, and 1 teaspoon salt, then cook stirring occasionally, until golden brown, about 5-7 minutes.
Add the tomato paste, paprika, and cumin; cook stirring often, until the tomato paste begins to stick to the pan, 1-2 minutes. Stir in the lentils, followed by the potatoes, roasted peppers, 6 cups water, and½ teaspoon each of salt and pepper.
Bring to simmer over medium-high, then cover, reduce to medium-low and cook, stirring occasionally, until the lentils are very soft, and the potato is tender, 25-30 minutes.
Remove the pan from the heat, stir in the lemon juice and let stand, uncovered, for about 5 minutes. Taste and season with salt and pepper. Serve sprinkled with cumin and parsley.