Preheat the grill to high. Put the milk, mustard powder and cheese in a small heavy-based saucepan and stir over a low heat until the cheese has melted and the mixture is smooth. Remove from the heat and stir in the breadcrumbs. Cool for 3–4 minutes, stirring occasionally, until thickened to a spreading consistency.
Meanwhile, arrange the slices of bread on the grill tray and toast on both sides under the grill.
While the bread is toasting, make the salad. Put the oil, red wine vinegar and poppy seeds in a salad bowl and season to taste with salt and pepper. Whisk to mix. Add the spinach, apples and celery, but do not toss the salad.
Top the toast with the slices of red onion, then spoon the cheese mixture over the top, spreading it out to cover the toast completely. Return to the grill and cook for 2–3 minutes or until the cheese mixture is golden brown and bubbling. Toss the salad and serve with the rarebits.