Melt in a soup pot over medium heat:
Add and cook, stirring, until tender but not browned, 8 to 10 minutes:
Sprinkle with:
Cook, stirring, 3 to 4 minutes more. Slowly whisk in:
Bring to a boil, whisking constantly. Reduce the heat to a simmer and cook until thickened, about 15 minutes. Puree the soup with an immersion blender (or in batches in a food processor or regular blender) until smooth. Return to the pot, bring back to a simmer, and stir in:
Reduce the heat to low and stir until the cheese is melted. Do not let the soup boil: If the soup is too hot, the oil will separate out of the cheese. Season to taste with:
Garnish with any of the following: