Melt 2 tablespoons butter in a pan over medium heat. Add onions and celery; cook and stir until golden brown, about 6 minutes. Remove from heat.
Place bread cubes, poultry seasoning, cooked onions and celery, salt and pepper in a mixing bowl. Pour in chicken stock. Toss until thoroughly blended. Let mixture rest for about 15 minutes to allow stock to absorb into the stuffing. Add egg yolk; stir until evenly incorporated.
Preheat oven to 190 C / Gas 5. Line a rimmed baking tray with greaseproof paper.
Slice turkey breast crosswise in 2cm slices down to the bone (do not cut through the bone). Divide stuffing mixture among the spaces between the slices. Transfer turkey breast to prepared baking tray. Brush with melted butter.
Roast in preheated oven until richly browned, 60 to 75 minutes. An instant-read thermometer inserted near the bone should read 65 C. Remove from oven and let rest 10 minutes before slicing.