Place the bagel halves, cut sides up on a baking sheet, spread over the garlic butter and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 10–15 minutes until crisp.
Meanwhile, heat the oil in a frying pan, add the onion and cook for 3 minutes. Add the chicken and cook for 5 minutes. Add the mushrooms cook for a further 5 minutes until tender. Add the sherry and allow to bubble, then stir in the crème fraîche and parsley and season with salt and pepper. Simmer, stirring, adding a little water if the mixture is too thick. Spoon over the baked bagels and serve.