Place clams in a large pot with enough water to cover. Stir in polenta. Let stand for about 20 minutes. Rinse clams thoroughly before cooking.
Meanwhile, heat the olive oil and garlic in a large pot over medium heat. When the garlic is fragrant, add the chorizo, onion and crushed chillies. Cook for 2 or 3 minutes, then pour in the beer. Let simmer for a few more minutes, then add the fish stock, tomatoes and rinsed clams.
Cover and simmer until all the clams have opened. Season with oregano, salt and pepper before serving.