Saute onions in high-heat oil. Add garlic. Stir to incorporate. Add chopped celery, dried or fresh thyme, and twist of pepper. Add broth to cover the veggies. Boil at medium-high heat, stirring occasionally, and cover with a lid to encourage veggies to steam and soften. (Optional: Add cubed potatoes for a creamier, heartier dimension to the soup. I typically prefer to highlight the flavor and natural creaminess of the onions and celery.)
Cook until celery is soft enough to fall apart when pressed on with a spoon. Turn off heat.
Use an immersion blender to puree into a creamy soup. You can play with this part of the process, leaving some vegetable bits or pureeing until entirely smooth. Alternatively, ladle into a blender, being careful not to over-blend or liquefy the soup.
Add 1-2 Tablespoons apple cider vinegar and a generous drizzle of olive oil. The apple cider vinegar adds an important acidic component that brings out the flavor of all the ingredients combined. A few squeezes of lemon juice or white wine vinegar would work well here, too, but I prefer the mildly sweet addition of the apple cider vinegar.
Salt and pepper to taste, being careful not to over-salt. Celery is a high-sodium vegetable, and highly flavorful, so it naturally needs less salt. Be mindful, too, of whether or not your broth contains salt already.
Optional additions include a drizzle of heavy cream, or a small knob of butter added prior to pureeing for a richer, creamier texture and flavor. (Today, I used none of these as I was craving a clean, crisp soup with the mild flavor of a good olive oil.)
Top with red pepper and black pepper and a sprinkle of thyme, or serve beneath grilled salmon, meat, or veggies for a surprising alternative to typical sauces or gravy. I keep this soup in the fridge for several days and use it as a flavor addition to nearly everything savory, including fried eggs and tomatoes on toast.