Split the tomatoes in half and scoop out the seeds and flesh. Trim the core tops with small knife. Season the tomato halves with salt and pepper.
Heat the extra, 3 turns of the pan, in a medium skillet over medium-high heat. Add the celery and mushrooms and cook 7 to 8 minutes until mushrooms are tender. Season the vegetables with salt and pepper and add vinegar to the pan. Turn off heat and stir in mustard and parsley to coat. Adjust the seasonings.
Divide the warm salad among the 4 tomato halves and serve.