Preheat the oven to 400°F (200°C).

Cut the squash in half from the stem and scoop out the seeds.

Drizzle 2 tablespoons of olive oil evenly over the insides of the squash, rubbing to coat. Sprinkle everything with salt and pepper.

Place the squash halves cut side up on a baking sheet, and bake for 40 minutes, or until fork-tender.

Meanwhile, heat 1 tablespoon of olive oil in a large pot over medium heat. Add onion, celery, red bell pepper and garlic. Cook, stirring often, until the vegetables are softened, for 5 minutes.

Pour in the broth, then allow the mixture to simmer, for about 10 minutes.

Scoop the flesh out of the acorn squash shells (leaving a thin layer if serving the soup in the shells if you desire). Add the squash flesh to the pot of soup and cook for an extra minute to blend the flavors.

Using a handheld immersion blender or working in batches with a blender, carefully blend the mixture until smooth. If using a regular blender, make sure to vent the top by removing the vent lid and using a thick towel there instead to absorb the steam.

Garnish with pumpkin seeds, crushed red pepper and thyme if desired. Serve hot (in a bowl or in the acorn squash shells).