Preheat oven to 425°F
Place tomatoes and onion on a rimmed baking sheet and nestle in the garlic cloves.
Drizzle vegetables generously with oil, 1/2 teaspoon salt and pepper
Roast for 30 -35 minutes until tops of tomatoes start to develop a char and onion starts to turn golden. Remove from heat.
In a large pot, squeeze the roasted garlic flesh out of the peel and discard peels.
Then, add the rest of the tomato onion mixture, being sure to scrape all of the delicious juices from the baking sheet.
To the vegetables, add the broth, beans, thyme and 1/2 teaspoon salt and bring mixture to a boil.
Reduce heat to simmer for ten minutes
Meanwhile, make the gremolata: in a small bowl, stir the fennel, parsley, sunflower seeds, lemon zest and garlic.
Add a pinch of salt, a dash of chile flakes and a drizzle of lemon juice and olive oil. Mix, taste and adjust with more salt, chile flakes, lemon juice or olive oil as desired.
To finish the soup, remove from heat and using a handheld immersion blender, blend until smooth. Taste and adjust salt, and if you wish, thin with a little extra broth.
Ladle into bowls and top with gremolata. If not using gremolata, give the soup a little squeeze of lemon juice for acidity.