Pat the pork dry and season liberally with salt and pepper. Heat the oil and half the butter in a heavy pan over medium heat.
Brown the pork loin, turning frequently, for 5-10 minutes or until dark golden brown. Transfer to a plate.
Add the shallots to the pan and cook, stirring frequently, for 5 minutes or until softened. Add the thyme and browned pork along with any accumulated juices. Pour in the stock. Cover and simmer for 30 minutes.
Turn the pork over and add the celery. Cover and cook for an additional 40 minutes.
In a small bowl, make a paste of the flour and remaining butter. Transfer the pork and celery to a platter and keep warm. Remove and discard the thyme. Whisk the flour-butter mixture into the simmering liquid a little at a time. Cook for 2 minutes stirring constantly. Stir in the cream and bring to the boil. Remove the pan from the heat.
Slice the pork and spoon over the sauce. Serve with fresh green peas, the celery and remaining sauce.