Heat the pot until hot then add oil and a sprinkle of salt. Sauté the garlic & onion in the hot oil. Before it browns, add the sliced potato. Sauté until the potato begins to cook.
Add the celery, carrots, pepper and salt. Sauté everything together for about 2 minutes. Then add the cubed squash.
Add the chicken broth (increase heat if needed) and bring to a boil. Add the lemon juice.
Cover the pot and bring to pressure. {If using a regular saucepan, put a weight on the cover to add some pressure; an upside down glass measuring cup works well.}
Set the timer for 6 minutes. Reduce pressure immediately. Smooth the soup with an immersion blender. Taste and adjust seasonings as needed.