Add butter to a large soup pot.
Place pot over medium heat and melt butter.
When butter is hot, add onions. Sauté until translucent.
Add thyme, roasted garlic and garlic purée. Continue to cook for about 3 minutes more. Reduce heat.
Add flour. Mix with whisk until flour is incorporated. Cook 2 minutes more.
Add chicken stock and marinara. Increase heat and bring to a boil.
Reduce heat to a simmer.
Add salt, pepper and heavy whipping cream. Allow to simmer for 15 minutes, skimming off the impurities that rise to the surface.
Remove from heat and cool slightly.
When cool enough to be safe, spoon soup into a food processor, working in batches if necessary. Process until smooth.
Strain soup back into pot.
Add 1/4 cup basil.
Taste. Add more salt and pepper, to taste, if needed.
Spoon into serving bowls. Garnish with 1 teaspoon of julienned basil per bowl.
Serve hot.