Heat oil in large pot over medium; cook onion 7-8 minutes until golden and caramelized, stirring occasionally.
Add garlic; cook 1 minute until fragrant, stirring to prevent sticking.
Add courgettes, broth, thyme, salt, and pepper; boil.
Reduce heat to medium-low; simmer 15 minutes until courgettes are very fork-tender.
Remove from heat; puree completely smooth with immersion blender, or carefully transfer to blender in batches.
Stir in cream cheese and lemon juice until fully incorporated and creamy.
Serve hot with basil.