How to Make It
Preheat the oven to 375°. Place the merguez on a rimmed baking sheet and bake until cooked through, about 25 minutes.
In a medium saucepan, combine 1 onion half with the whole garlic cloves, the carrot and the lentils. Add enough cold water to cover the lentils by 2 inches. Bring to a boil over high heat, then cover and simmer over moderately low heat until the lentils are just tender, 25 to 30 minutes. Drain; discard the onion, carrot and garlic.
Meanwhile, in a large enameled cast-iron casserole or Dutch oven, heat the olive oil over moderately high heat. Finely chop the remaining onion half and add it to the casserole along with the chopped garlic and a pinch of salt. Cook, stirring frequently, until the onion is softened. Add the squash, season with salt and cook, stirring frequently, until golden. Add the cumin, paprika, Marash pepper and harissa and cook, stirring, for 1 minute. Stir in the vegetable stock and tomatoes and bring to a boil. Reduce the heat, cover and simmer until the squash is tender, about 15 minutes.
Stir the lentils, jalapeño and lemon juice into the casserole and season with salt and pepper. Transfer to a platter and top with the sausage. Serve with lemon wedges.
Marash and Aleppo pepper are available at Middle Eastern markets and online at vannsspices.com.