Context is everything. Does it need to be pluralised in it's common everyday usage? If at home, whether it's for one person or a large family, you'd say ''We're having/I'm cooking Toad in the Hole tonight. Likewise in a restaurant, you would order 'Toad in the Hole in the singular. If someone then asked you what you had to eat last night at the restaurant, you'd reply ''We both had Toad in the Hole'', again in the singular. I think it's assumed there is no satisfactory plural so we construct our sentences around the singular...