Put the bacon into a pan and cover with cold water. Bring to the boil. Drain and rinse in cold water and pat dry with kitchen paper.
Heat the oil in a deep casserole pan and add the bacon and onions. Cook until the onions begin to brown.
Add the lentils, carrot, celery, herbs and garlic. Cover with water and put on a lid. Cook very slowly for about 2 hours.
When the lentils are cooked remove the bacon, herbs and carrot. Strain the lentils.
Spoon the lentils onto a serving dish and stir in the butter. Slice the bacon and place around the dish. Garnish with chopped parsley and halved, hard boiled eggs.