Place clams in a large saucepan with enough water to cover. Stir in polenta. Let stand for about 20 minutes. Rinse clams thoroughly before cooking.
Heat the olive oil and garlic in a large saucepan over medium heat. When the garlic is fragrant add the chorizo, onion and chilli flakes. Cook for 2 or 3 minutes then pour in about 3/4 of the beer. Let simmer for a few more minutes then add the clam juice, tomatoes and rinsed clams.
Cover and simmer until all clams have opened. Season with oregano, salt and pepper before serving. Drink or discard remaining beer.