Add butter and olive oil to the pan, once hot add the onions. Saute until fragrant, 2 minutes.
Add the leeks and saute until softened, stirring frequently and very lightly browning, 5 to 7 minutes.
Add garlic and saute for 30 seconds.
Transfer sauteed vegetables to the slow cooker.
Add potatoes, vegetable broth, salt, bay leaf, and thyme to the slow cooker.
Place lid on slow cooker and cook on “High” setting for 4 to 6 hours or “Low” setting 6 to 8 hours, or until potatoes are fork tender.
Remove and discard bay leaf and thyme sprigs.
Use a hand immersion blender to puree the soup until just smooth, or transfer to a blender and puree. If soup is removed from the slow cooker, transfer back to the pot.
Add in heavy cream and black pepper, whisk to combine.