In a large pot or Dutch oven, cook the diced bacon or sausage over medium heat until crispy or browned. Remove the cooked meat with a slotted spoon and set it aside, leaving the rendered fat in the pot.

Add the butter or oil to the pot (if needed). Add the chopped onion, carrots, celery, and leek. Cook, stirring occasionally, until the vegetables are softened, about 8-10 minutes. Add the minced garlic and cook for another minute until fragrant.

Add the cubed potatoes and the vegetable or chicken broth to the pot. Stir in the bay leaf. Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for 15-20 minutes, or until the potatoes are very tender and easily pierced with a fork.

If you prefer a completely smooth soup, you can use an immersion blender to puree the soup directly in the pot until it reaches your desired consistency. Alternatively, you can carefully transfer the soup in batches to a regular blender. Be very cautious when blending hot liquids! If you like a chunkier soup, you can skip this step entirely or just blend a portion of it.

If using, stir in the heavy cream or milk. Add the cooked bacon or sausage back to the pot (if using). Season the soup with salt, black pepper and marjoram to taste. Heat through gently, but do not boil.

Remove the bay leaf. Ladle the soup into bowls and garnish with fresh parsley or chives, if desired.