Preheat oven to 425 degrees. Place tomatoes on a rimmed baking sheet and toss with 3 Tbsp olive oil then season with salt and pepper (place Roma halves cut side upright).

Place onion slices and garlic on a rimmed half sheet and toss brush with remaining 1 1/2 Tbsp olive oil on all sides, sprinkle with salt and pepper.

Place baking sheet with tomatoes and half sheet with onions in oven side by side and roast garlic 15 minutes (or until just golden) and remove, roast onions 30 - 35 minutes (until edges are golden brown) and remove, and tomatoes 40 - 45 minutes until slightly browned.

Pour tomatoes, onions and garlic into a large pot. Add vegetable broth and about 3/4 of the basil. Season with salt and pepper to taste.

Bring to a boil, cover, then reduce heat and simmer 20 minutes.

Blend soup with an immersion blender or in small batches in a blender (only fill blender half full and remove center insert and cover with a kitchen towel). Serve warm topped with remaining basil (chopped).