Heat the pressure cooker and add the oil plus 1 Tbsp butter.
Season the chicken legs then fry them turning occasionally for about 5 minutes so they're browned all over.
Take out the chicken, add another Tbsp of butter and fry the onions until soft.
Gradually stir in the garlic, mushrooms, wine & tomato purée then turn up the heat and reduce the liquid for about 5 minutes.
Add the thyme and stock then return the chicken to the pan. Cook under pressure for 20 minutes (high) / 25 minutes (medium) according to manufactures instructions.
Remove from heat and allow to de-pressurize on its own. Take out the chicken and swiftly reduce the liquid until it's sticky. Return the legs and coat each with the sauce before serving.