Bring roughly 500 ml water to a boil in a pot over medium-high heat. Add the rice, stir, reduce heat to low, cover, and simmer for roughly 20 minutes until tender. While the rice cooks, rinse the bell peppers, cut off the tops, and carefully remove all seeds and white membranes.
In a bowl, combine the minced meat, mustard, egg, finely chopped onion, approximately 4-5 tablespoons of cooked rice (adjust based on pepper size), fresh chopped parsley, salt, and pepper. Mix thoroughly into a thick, sticky paste. Divide the mixture evenly between the peppers and replace the pepper lids.
In a saucepan, bring the vegetable stock to a boil over medium-high heat. Stir in the tomato paste and ketchup until well combined, then add the paprika powder. Place the stuffed peppers in the sauce, reduce heat to low, cover with a lid, and simmer for 30 to 40 minutes.
Remove the peppers from the saucepan with a slotted spoon. Stir the cream into the remaining sauce and season with salt, pepper, and additional paprika powder to taste. If desired, dissolve a little cornstarch in cold water and stir into the sauce, simmering for 1-2 minutes until thickened.
Reheat the remaining cooked rice in the microwave for 2-3 minutes. Plate the peppers with rice and pour the sauce over the top.