In a skillet, lightly sauté the onions and garlic until translucent. (Optional but recommended for deeper flavor.)
Add potatoes, sautéed onions, garlic, salt, pepper, and chicken broth to the crock pot.
Cover and cook on LOW for 6 hours or HIGH for 3 hours, until potatoes are very tender.
In a small bowl, whisk together the cold milk and flour until smooth. Stir into the soup.
Use an immersion blender (or transfer soup in batches to a blender) to puree the mixture until completely smooth and creamy.
Stir in the heavy cream and Velveeta cheese. Cover and let sit on warm for 10-15 minutes until the cheese is fully melted, stirring occasionally.
Taste and adjust seasoning if needed.
Serve hot, topped with crispy bacon, green onions, or extra cheese as desired.