Cook the rice in plenty of boiling salted water for about 20 minutes. Add the peas 5 minutes before the end of the cooking time. Drain the corn. Wash the tomatoes and cut off a lid. Remove the seeds from the tomatoes with a spoon.
Cut the lid small. Pass the tomato pips and chopped flesh through a sieve. Stir the liquid obtained with sour cream and cornstarch until smooth. Season with salt and pepper.
Put rice on a sieve, drain and mix with the corn. Wash the thyme and dab dry. Season tomatoes from inside with some thyme leaves, salt and pepper and fill with the rice mixture.
Pour parmesan cheese on the filling. Place the tomatoes in an ovenproof casserole dish and pour the sauce over them. Bake in a preheated oven (electric: 175 °C/ gas: level 2) for about 10 minutes. Sprinkle the remaining thyme leaves on the sauce.
Garnish tomatoes with basil as desired.