For the steak sandwich, rub the steak with the olive oil and season well with salt and freshly ground black pepper.
Heat a frying pan until smoking hot, add the steak and cook for three minutes on each side, or for a minute longer if you like your steak well-done.
Remove the steak from the pan and add the onions.
Cook the onions over a gentle heat until caramelised.
Place a slice of griddled bread onto a serving plate and top with the caramelised onions and steak.
For the chutney, place the grated carrot and vinegar into a small pan and cook for a few minutes, or until the vinegar has reduced to one tablespoonful.
Add the chilli flakes, ground allspice, cinnamon, honey and tomato purée and cook for two minutes, or until the mixture becomes jammy. Season, to taste, with salt and freshly ground black pepper.
Spoon the chutney on top of the steak sandwich.
For the crisps, place the vegetable oil into a deep, heavy-based saucepan and heat until a small cube of bread sizzles and turns golden-brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.).
Very finely slice the celeriac on a mandolin and dry in a clean tea towel.
Carefully drop the celeriac slices into the hot oil and fry for four minutes, or until crisp and golden-brown.
Carefully remove with a slotted spoon and drain on kitchen paper. Scatter with sea salt and serve with the steak sandwich.