Stuffing Stuffed Turkey Breast Roast by hip pressure cooking

– 2 tablespoons un-salted butter
– 2 celery stalks, chopped
– 2 small red onions, chopped
– 2 cloves garlic, minced
– 2 sprigs fresh sage, chopped (about 2 tablespoons )
– 1 1/2 teaspoons salt, divided
– 1 pinch pepper
– 2 cups (200 g) plain dried breadcrumbs
– 1 cup (250 ml) whole milk
– a few springs parsley, chopped (about 2 tablespoons )
– 1 tablespoon olive oil
– 2 1/2 -3 pound (1 – 1.5 k) boneless & skinless turkey breast, butterflied (sliced so that it is suitable for rolling) and pounded to an even thickness
– 2 teaspoons grainy mustard
– 2 cups (500 ml) vegetable stock