"This recipe was originally a higher-fat recipe. But with age and ailments comes restraint and tradeoffs. To try the original recipe, use butter instead of extra-virgin olive oil and whole milk instead of skim. I am not much of a celery fan, especially in mild-flavored foods, but if you like celery, add small diced celery to taste along with the onion, garlic and potato. Serve with crackers of choice. I personally don't use crackers. I feel they detract from enjoying the mild desired flavor and creamy dreaminess."
Heat olive oil in a saucepan over medium heat. Add potato, onion, and garlic to the hot oil, reduce heat to medium-low, and bring the mixture to a simmer. Place a lid on the saucepan and cook the mixture at a simmer, stirring occasionally, until the onion and potato soften, 5 to 7 minutes.
Season the potato mixture with salt and black pepper.
Stir flour into the mixture to make a roux. Increase heat to medium and stir the mixture constantly to cook the flour, 3 to 4 minutes.
Pour reserved oyster liquid into the potato mixture. Increase heat to medium-high.
Slowly stream skim milk into the saucepan while stirring continually; cook and stir until the mixture begins to thicken and bubble, 5 to 7 minutes. Season again with salt and black pepper as needed.
Reduce heat to medium-low. Add oysters to the saucepan; continue to cook and stir until the mixture is thick and the oysters are just cooked through, about 3 minutes.
Turn off the heat and continue to stir until you are sure soup will not boil after covered. Cover and let stand 10 minutes for the flavors to blend.