Cook 1 slice of bacon til crispy. Set aside as garnish for later.
Heat a large saucepan over medium high heat.
Add onion, celery and remaining bacon and cook until onions soften.
Add stock and potatoes. Bring to the boil, reduce heat and simmer covered for 10-15 mins, or until potatoes are tender
Blend flour in half a cup of milk, then once blended add remaining milk and mix.
Slowly pour into the soup, increase heat and stir constantly until soup boils and thickens.
Remove from heat and stir in half the cheese and half the green onions
Season to taste with black pepper
Serve topped with remaining green onions, cheddar and bacon.