Heat the oil in the casserole then, with the heat at medium, brown the sausages evenly all over, taking care not to split the skins by turning them over too soon.Next, using a slotted spoon, transfer them to a plate while you brown the diced bacon along with the garlic and shallots. Now crush the juniper berries very slightly without breaking them – just enough to release their flavour. Return the sausages to the casserole, pour in the wine and add the berries, then thyme and bay leaves. Now season lightly, bring it all up to a gentle simmer, put a lid on the casserole, turn the heat as low as possible and let it all simmer gently for 30 minutes.After that, add the mushrooms, stirring them in well, then leave everything to cook gently for a further 20 minutes – this time without the lid so the liquid reduces slightly. To finish off, remove the sausages and vegetables to a warm serving dish, mix the flour and the mustard powder with the softened butter until you have a smooth paste and whisk this, a little at a time, into the casserole. Let everything bubble for a few more minutes, then take the casserole off the heat, return the sausages to the casserole, whisk in the redcurrant jelly – and it's ready to serve.