Wash mussels very carefully, rinsing under running water several times to remove dirt. Heat oil in a pan over medium heat, add 1 peeled garlic clove and mussels and cook with the lid on, until they open. Remove from the heat, drain the cooking liquid and strain through a sieve. Set aside.
Heat oil in a pan over medium heat and sweat minced onion without browning it. Add rice and sauté for 2 minutes until translucent. Pour in white wine and let it evaporate. Remove rice from the heat and set aside.
Peel and slice the potatoes. Boil in salted water until soft. Sprinkle bottom of a baking dish (the "tiella") with salt, add julienned onions and chopped parsley, and drizzle with oil.
Add remaining 4 garlic cloves (peeled and chopped), thyme, cherry tomatoes (washed and halved), 1/2 cup grated Pecorino and cover with half the potatoes. Add sautéed rice and a bit of parsley.
Shell half the mussels and stir in. Make another layer of tomatoes and potatoes.
Add 1/2 cup of the mussels' cooking liquid and 2/3 cup fish broth to the "tiella". Sprinkle with the remaining Pecorino, cover with foil and cook in a preheated 350°F oven for 18 - 20 minutes, until the rice is cooked.
This dish can be served in its baking dish or in individual plates garnished with open mussels.
A dish with certainly marked and lingering flavor and fragrances therefore we propose a Malvasia from Puglia, a well-structured white wine with pale greenish highlights and that is fresh, tasty, delicate and well-balanced. Gravina Malvasia D.O.C.
Always use boiling hot broth before placing it in the oven.