Heat a medium skillet over medium heat. Add 2 tablespoons butter and cumin seeds and cook, stirring, until fragrant, 1 to 2 minutes. Stir in leeks, carrots, shallots, ginger and 1/8 teaspoon salt. Cover and cook, stirring occasionally, until the carrots begin to soften, about 10 minutes. Stir in jalapenos, coriander and turmeric and cook, stirring, 1 minute more. Stir in tomatoes and 1 cup of the reserved cooking liquid (if there isn't enough, make up the difference with water). Bring to a simmer, stirring occasionally, then cook for 5 minutes. Stir in the reserved lentils and simmer until they are soft and tender but not falling apart, 3 to 5 minutes. If the mixture seems dry, add more cooking liquid (or water). Stir in roasted peppers. Season with the remaining 1/2 teaspoon salt and pepper. Divide the lentil stuffing among the squash (a generous 1 cup per half). Serve garnished with a dollop of yogurt.