Preheat oven to 375°F. Measure out all ingredients. Wash and dry the bell peppers and parsley. Dice the onion and shred the carrots. Rinse and cook rice according to the package instructions. Cut off the peppers tops. Do not discard them. Remove all the seeds.
Rinse the rice a few times. Transfer rinsed rice to a small pot with water, bring to a boil over medium-high heat, then reduce to low, cover, and simmer until the water is fully absorbed and the rice is slightly al dente in the center. Transfer the rice into a large bowl to cool down.
While the rice cooks, carefully slice off the tops of the bell peppers, remove the seeds and inner ribs, and set the hollowed peppers aside.
In a large bowl, add together rice, vegetables, ground beef, chopped parsley, salt, thyme and paprika. Mix it until all ingredients are combined. Add ½ cup of water to the meat and mix it again. Stuff each pepper with the juicy meat mix. Arrange each stuffed pepper in a baking dish standing up and cover with the tops.
Heat tbsp olive oil in a non-stick frying pan over medium heat. Add the diced onion and cook for about 3 minutes until translucent. Add the shredded carrots and cook, stirring occasionally, for about 5 minutes until soft. Add the sautéed vegetables into the large mixing bowl with the cooked rice and allow the mixture to cool down slightly.
Preheat your oven to 375℉. To the cooled rice and vegetable mixture, add the ground meat, dried thyme, paprika, black pepper, chopped parsley, 1 teaspoons of salt,and ¼ cup of cold water. Mix thoroughly until well combined. Evenly stuff each hollowed bell pepper with the meat mixture, packing gently. Arrange the stuffed peppers upright in a deep baking dish (they should fit snugly side-by-side).
Boil 3 cups of water. In a medium bowl, whisk 1 cup of boiling hot water together with the tomato paste and ¼ teaspoon of salt (or bouillon) until completely smooth. Carefully pour the tomato mixture into the baking dish around the peppers (avoid pouring directly over the tops). Add the remaining 2 cups of hot water, bay leaf, and 3 tablespoons of sour cream directly into the pan liquid. Gently stir the liquid around the base of the peppers until the sour cream blends into a light, creamy sauce. Ensure the sauce reaches at least ¾ of the way up the sides of the peppers.
Cover the baking dish tightly with heavy-duty foil or a fitted lid. Bake for 1 hour until the peppers are completely soft and tender and the meat is fully cooked through (165℉ internal temperature). Remove the cover and bake for an additional 5-10 minutes to lightly brown the pepper tops and reduce the sauce slightly. Remove from the oven and let rest in the baking dish for 10 minutes before serving. Serve with freshly chopped dill parsley and a dollop of sour ream.