Heat a frying pan until hot, then add the olive oil and onions. Fry the onions for 3-4 minutes, until just beginning to soften, then season well with salt and freshly ground black pepper and add the mint. Stir well, then remove from the heat and set to aside to cool.
Use the bacon to line the base and sides of a 1 litre/2 pint pudding basin, leaving some of the bacon slices hanging over the top of the bowl.
Place a little of the sliced mutton into the bottom of the bacon-lined bowl, then season with salt and freshly ground black pepper.
Top the layer of mutton with some of the onions, then repeat with alternate layers of mutton and onions, until the bowl is three-quarters full.
Fold the excess bacon over the filling to cover and press down firmly.
Cover the top with aluminium foil and place the bowl into a deep baking dish three-quarters-filled with hot water (this is a bain-marie).
Transfer to the oven to bake for two hours, then remove and allow to cool for 15-20 minutes.
Place a couple of cans of tomatoes or beans (or any reasonably weighty objects) on top of the foil over the terrine and place into the fridge until cold.
Meanwhile, for the beetroot chutney, heat a frying pan until hot, then add the olive oil, red onion, garlic and ginger. Fry for 2-3 minutes, until just softened.
Add the mustard seeds and cook until they pop.
Add the beetroot, sugar and vinegar and cook for 4-5 minutes, or until the chutney has thickened. Season, to taste, with salt and freshly ground black pepper and allow to cool.
To serve, turn the terrine out of the basin. Cut out wedges of terrine and place onto plates with a spoonful of chutney and a hunk of crusty bread alongside.