Begin by heating a large skillet over medium heat. Add 1 tablespoon unsalted butter and 1 tablespoon olive oil to the pan. Once the butter melts, it will create a beautiful base for your soup.
Add ½ cup diced yellow onion to the skillet. Stir occasionally, cooking until fragrant, about two minutes. You want the onion to soften and become translucent.
Next, add 4 cups sliced leeks to the skillet. Cook them until they are softened, stirring frequently, for about five to seven minutes. You’re aiming for them to be tender and slightly caramelized, which will deepen the flavor.
Incorporate 1 tablespoon minced garlic into the mixture and cook for an additional thirty seconds. You’ll notice an incredible aroma as the garlic releases its scent.
Carefully transfer the sautéed vegetables into the slow cooker. This will serve as the flavor foundation for your soup.
Add 3 cups peeled and cubed russet potatoes, 4 cups unsalted vegetable stock, 1½ teaspoons kosher salt, 1 bay leaf, and 3 sprigs thyme to the slow cooker. Everything should be evenly distributed.
Cover the slow cooker and set it to cook on the “High” setting for four to six hours or on “Low” for six to eight hours. You’ll know it’s ready when the potatoes are fork-tender.
Once cooked, remove and discard the bay leaf and thyme sprigs. This step is crucial for avoiding any unwanted flavors in your soup.
For a creamy finish, use a hand immersion blender to puree the soup until it’s just smooth. If you don’t have an immersion blender, carefully transfer the soup to a blender in batches to puree.
Once pureed, return the soup to the pot if you used a blender. Stir in ½ cup heavy cream and ¼ teaspoon black pepper. Whisk to combine all the flavors.
Finally, taste the soup and adjust the seasoning if needed. Garnish with a drizzle of cream and 1 tablespoon chopped chives before serving.
