Soak the clams in water with 1 handful of coarse salt for 1 hour. Drain.
Heat 4 tablespoons of extra virgin olive oil in a saucepan over medium heat: add the tomato passata and cook for 5 minutes.
Add the chickpeas and season with salt; stir and cook for 10 minutes. Dissolve the rice flour in the stock, then add to the saucepan and cook for 15 minutes.
Meanwhile, in another saucepan cook the garlic and chilli pepper in the remaining oil until sizzling. Add the clams, shake the pan and cover. Cook until the clams are open, between 5 and 10 minutes. Remove from heat and shell half of the clams. Set aside the remaining clams and keep them warm.
Strain the liquid released by the clams and add to the beans. Add the shelled clams, stir and cook 5 to 10 minutes longer.
Transfer onto serving plates, sprinkle with a mix of parsley, thyme and lemon zest. Decorate with the reserved clams ans serve.
You can make this soup a few hours ahead - it actually allows the flavours to meld better.