Halve the tomatoes diagonally. Using a small teaspoon remove the seeds and juice and reserve.
Place the breadcrumbs and parsley into a small bowl and add the reserved tomato juice. Season with salt and pepper.
Heat the butter and cook the onion until soft. Add the breadcrumb mix and thyme and cook for a few minutes. Stir in half the cheese and place the mixture into the halved tomatoes. Top with the remaining cheese and bake for 25 minutes. Serve immediately.