Heat olive oil and butter in a large pot over medium heat.
Add onions and celery, then season lightly with salt and pepper. Sauté until veggies start to get tender, stirring occasionally (about 10 minutes).
Stir in garlic and cook just until fragrant, stirring constantly (about 30 seconds).
Add broth, potatoes, bay leaf, and basil to pot. Cover and bring to a boil over high heat. Remove lid, reduce heat, and simmer until veggies are tender, stirring occasionally (about 10 minutes).
Mix parsley into soup, then simmer for a couple minutes.
Remove soup from heat, discard bay leaf, then purée until totally smooth using immersion blender (or with a traditional blender, in batches). Season generously to taste with salt and pepper.
Serve soup hot. Enjoy!
Notes
How to prepare the celery quickly: rinse well under running water, then trim the tips from the leafy end and any browned or damaged parts, leaving the root end intact. Grasp the head to hold the ribs together, then thinly slice the celery (about ¼-inch thick) through the whole bunch. Make sure to use a sharp knife and to watch your fingers. When you get to the root end, toss it out. You can use the leaves and the stalks for this recipe.If you'd prefer a thicker soup, continue to simmer for a few minutes after pureeing, stirring frequently.This recipe makes about 8 cups of soup, which is 8 small servings (1 cup), 6 average-size servings (1 ½ cups) or 4 large servings (2 cups).Check out the info before this recipe card for more tips and variations!