Rinse the split peas, then soak them in cold water for at least 3 hours, but it’s even better if you leave them overnight. This will help them soften more quickly.
After soaking, heat the oil (or butter) in a pot and sauté the finely chopped onion and minced garlic.
Drain the soaking water from the split peas and add them to the pot. Pour in enough water to cover them by about twice their volume. Season with salt and ground black pepper, and add a bay leaf. Cook until completely tender. Keep an eye on it, as split peas tend to boil over.
Once the peas are soft, remove the bay leaf and blend the soup until smooth and creamy using an immersion blender.
When serving, stir in a spoonful of sour cream.