Wash and scrub clams and mussels. Remove beards from mussels and refrigerate until ready to assemble dish. Discard any clams that are open and do not close, or that do not smell fresh.
In a large 14-inch stainless steel sauté pan with lid, add oil, heat till lightly smoking then add garlic, shallots, saffron, fennel, thyme, and chorizo. Sweat but do not brown, about 2 to 3 minutes. Add mussels, clams, and white beans all together. Toss with a spoon. Deglaze with white wine. Cook for 1 minute then add clam juice and cover with lid. Do not reduce heat. Allow to cook for 3 minutes or until clams and mussels open.
To finish, after clams and mussels are open, add basil, arugula, cherry tomatoes and season with salt and pepper. Cover with lid and allow ingredients to cook together for 1 minute longer. Serve immediately.
Cooking this dish requires only 7 to 8 minutes; do not start until ready to serve.
Great accompaniment to this dish is crusty Chibatta bread rubbed with garlic, brushed with olive oil and grilled. Great for mopping up wonderful broth from clams and mussels.