Preheat oven to 200°C. Line an oven tray with baking paper.
Arrange tomatoes, cut-side up, on tray with garlic. Drizzle with half oil, season. Bake 20-25 mins until collapsing.
Meanwhile, in a large heavy-based saucepan, heat remaining oil on medium. Saute celery and onion 4-5 mins until onion is tender.
Stir in tomato, garlic (squeezed from skin) and stock. Bring to boil. Reduce heat to medium. Simmer 10 mins. Stir basil through. Cool slightly. Using a hand blender, puree mixture until smooth.
In a large saucepan, melt butter on medium. Add flour and cook, stirring, 1 min. Remove from heat. Gradually stir in milk until smooth. Return to heat. Cook, stirring constantly, until mixture boils and thickens. Simmer 3 mins, stirring.
Gradually add tomato puree, stirring constantly until smooth. Bring to boil on medium. Reduce heat to low, simmer 1-2 mins. Season. Serve with extra sliced basil leaves, prosciutto crumbs (see cook's tip), a drizzle of olive oil and plenty of black pepper.
