Gather fresh ingredients and prepare vegetable broth using the vegetable bouillon and water.
To a large soup pot over medium-high heat, add onion, garlic, celery, dried thyme, chili flakes, salt, black pepper, butter, and olive oil on medium heat. Cook until vegetables have started to soften, about 8 minutes.
Reduce heat to medium-low. Add apple cider vinegar, the diced potatoes and rinsed frozen corn. Cook with the garlicky veggies for about 10 minutes until the potatoes are about half cooked, still with a little bit of bite to them.
Add vegetable broth and room temperature milk, scraping up any bits of veggies from the bottom of the pot. Stir to combine.
Simmer (do not boil) over medium-low heat, stirring occasionally until potatoes are fully cooked, about 20 minutes.
Remove 2 cups of broth and whisk in the flour until smooth. Add back to the pot and stir to combine. Once the soup has simmered and thickened to your liking, test the vegetables. When the potatoes are done, the soup is done!
Mix in the minced fresh parsley or sprinkle it on top as garnish, and serve!