Preheat oven to 400 degrees.
Place acorn squash on a baking sheet and drizzle with oil. Toss chunks to coat evenly, then roast for 20 to 30 minutes, until soft.
Heat 3 tablespoons oil in a medium pot, add onions and sauté for 10 minutes over a medium heat. Add garlic, reduce heat and cook for another few minutes; then add cumin, coriander and cinnamon. Sauté spices for a few minutes. Add lentils and 1 quart just-boiled water. Cover and simmer for 10 minutes.
Once lentils soften, add ¾ of the squash, vegetable stock, lemon juice and 1 teaspoon sea salt; simmer for another 10 minutes, or until lentils are fully cooked. Season with more salt and pepper, to taste. Divide soup between bowls and top with remaining squash.