01 - Melt butter in a large pot over medium heat. Add leeks, onion, and garlic. Sauté for 5–6 minutes until soft and fragrant, taking care not to brown.
02 - Add diced potatoes to the pot and cook for 2–3 minutes, stirring occasionally to coat with butter.
03 - Pour in vegetable broth, salt, pepper, and nutmeg if using. Bring to a boil, then reduce heat and simmer uncovered for 20–25 minutes until potatoes are completely tender.
04 - Remove pot from heat. Using an immersion blender, purée soup until smooth and creamy. Alternatively, blend in batches using a countertop blender and return to pot.
05 - Stir in milk and heavy cream, then add grated Irish cheddar. Heat gently over low heat, stirring continuously until cheese is fully melted and soup is velvety.
06 - Taste soup and adjust salt and pepper as needed. Serve hot in bowls, garnished with fresh chives and additional cheddar if desired.