Dice the shallots and bacon finely, and gently fry for a while in a little olive oil.
Blanch the lentils in boiling salted water for 2 minutes, drain, rinse with cold water, and add to the bacon and shallots.
Pour, in the stock, bring to the boil, and reduce somewhat before adding the cream and then reducing a little more until thickened.
Add Dijon mustard, balsamic vinegar, salt and pepper to the lentils according to taste.
Immediately before serving, stir in the chopped parsley.
Red Wine Butter Sauce:
Dice the shallots finely, and sauté gently in butter with the sprig of rosemary.
Add the sugar, and continue cooking until it is lightly caramelised.
Pour in the Rioja, reduce over a low heat until only about one-third of the liquid remains, and then pass through a fine sieve.
Just before serving, bind by stirring in the cold butter using a whisk.
Sheatfish:
Season the fish medallions with salt and pepper.
Heat some corn oil in a non-stick frying pan, and put the pieces of fish in the pan with the skin side downwards.
Fry until the underside becomes quite crispy, and then turn the medallions over, reduce the heat, and allow the fish to cook for a little longer until it has a glassy appearance.
To Serve:
Place some lentils in the centre of each plate, top with three sheatfish medallions, and put some of the Rioja butter next to the lentils.