Preheat the oven to 425°F (220°C).
Place the pumpkin cubes on a large baking sheet. Drizzle with 1 tablespoon olive oil and sprinkle with 1/2 teaspoon salt and 1/4 teaspoon black pepper. Toss to coat evenly.
Roast the pumpkin in the preheated oven for 25-30 minutes, turning halfway through, until tender and caramelized. Remove from oven and set aside.
While the pumpkin roasts, heat the remaining 1 tablespoon olive oil in a large pot over medium heat.
Add the diced onion and sauté for 5-7 minutes until translucent and softened.
Add the minced garlic and grated ginger; cook for 1 minute until fragrant.
Stir in the ground cumin, coriander, cinnamon, turmeric, and cayenne pepper. Cook for 1 minute to toast the spices.
Add the rinsed lentils, vegetable broth, diced tomatoes with their juices, tomato paste, remaining 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Stir to combine.
Bring the stew to a boil, then reduce heat to low and simmer uncovered for 25 minutes, stirring occasionally, until lentils are tender.
Stir the roasted pumpkin into the stew and simmer for an additional 5 minutes to meld flavors.
Remove from heat and stir in the lemon juice.
Ladle the stew into bowls and garnish with chopped fresh cilantro before serving.