In a medium skillet over medium heat, warm oil. Add onion and cook, stirring, until golden, about 8 minutes. Add garlic, ginger, cumin, salt, coriander, and red pepper flakes; cook, stirring, until fragrant, about 1 minute longer. Spoon into a 5- or 6-qt slow cooker.
Add squash, lentils, and broth to slow cooker, stirring well to combine. Cover and cook until lentils and squash are tender, about 2 hours on High or 4 hours on Low. Stir in chickpeas and tomatoes. Cover and cook 30 minutes on High. Stir in chopped parsley. Ladle evenly into 6 shallow bowls and dollop with yogurt; sprinkle with paprika and peanuts. Garnish with parsley sprigs.
Per serving: 11/2 cups tagine, 2 tbsp yogurt, and 1 tbsp peanuts