Preheat your oven to 400°F (200°C). Spread the cubed pumpkin on a baking sheet, drizzle with a bit of olive oil, and toss to coat. Roast for about 25 minutes until the edges are golden and the pumpkin is tender when pierced with a fork.
While the pumpkin roasts, heat a splash of olive oil in a large pot over medium heat. Add the chopped onion and cook, stirring often, until it turns translucent and fragrant, about 5 minutes.
Add the minced garlic, ground cinnamon, and cumin to the onion. Stir for about 30 seconds until the spices are fragrant and the mixture is well combined.
Pour in the vegetable broth and bring the mixture to a gentle simmer. Rinse the lentils and add them to the pot. Stir well, cover, and let simmer for 20-25 minutes until the lentils are tender and the flavors meld together, stirring occasionally.
Once the roasted pumpkin is ready, add it to the simmering stew, breaking it up slightly with the back of a spoon so it melds into the broth.
If using, stir in the canned diced tomatoes for extra richness and slight acidity. Season with salt and pepper to taste, then cook for another 5 minutes to heat through and allow flavors to develop.
Adjust the stew’s consistency by adding more broth if it’s too thick, or simmer longer if you prefer it thicker. The stew is ready when the lentils are tender, and the flavors are well combined with a warm, earthy aroma.
Serve the stew hot, garnished with a sprinkle of fresh herbs or a drizzle of olive oil if desired, for a hearty and comforting meal.