Melt the butter in medium saucepan over medium-high heat. Add the onion, celery, carrot, salt, and pepper, and cook until the vegetables are starting to soften and brown, about 8 to 10 minutes, stirring occasionally.
Add the garlic and roasted red bell pepper, and cook 1 minute, stirring constantly.
Add the lemon juice and scrape up any brown bits from the bottom. Cook until the lemon juice is evaporated, about 1 minute.
Stir in the flour and cook 1 minute, whisking constantly.
Whisk in the warm stock, Worcestershire, mustard powder, and paprika.
Bring the soup to a simmer and cook until the vegetables are completely tender, about 5 minutes, stirring occasionally.
Remove from heat and carefully puree the soup in batches in a blender or using an immersion blender.
Transfer the soup back to the pot and bring to a simmer again. Turn heat off and add the cheese, stirring until completely melted.
Ladle the soup into serving bowls and garnish with fresh parsley if desired.