Cut the pork tenderloin into slices about 1cm thick.
Mix the mustard with the minced garlic and some freshly ground black pepper, then rub it over the pork slices. Set aside while you prepare the vegetables.
Heat the olive oil in a large frying pan or casserole dish over a medium heat.
Add the shallots and celery, and cook for about 5 minutes until softened.
Add the pork slices and lemon juice. Cook for 1 minute, then turn the pork over and cook for another minute to lightly seal the meat.
Pour in the water or stock, then add the lentils and chopped greens.
Cover and simmer gently for 5–8 minutes, until the sauce has thickened and the greens are tender.
Take care not to overcook the pork, as it can become dry.
Serve warm, sprinkled with chopped fresh parsley.