Preheat your oven to 350°F (175°C). It’s crucial to get it ready beforehand, so your Kapusta bakes evenly and gets that lovely golden finish.

Grab a medium saucepan and place the sliced cabbage inside. Cover it with water and bring it to a boil over high heat. Let it cook until the cabbage becomes tender, about 10 minutes. Once tender, drain the cabbage and set it aside; this process softens the cabbage and enhances the dish's overall texture.

In a large skillet, melt 4 tablespoons of butter over medium heat. Add in the chopped onions and the sliced portobello and white button mushrooms. Sauté these vegetables until they're tender and slightly golden, around 5 to 7 minutes. The butter will give the veggies a rich flavor that will permeate the dish.

In a 9x13-inch baking dish, combine the drained cabbage, your sautéed onion and mushroom mixture, the drained sauerkraut, white sugar, smoked paprika, thyme, along with salt and pepper to taste. Stir everything gently but thoroughly to ensure an even distribution of flavors.

Cut the remaining 2 tablespoons of butter into small pieces and distribute them over the top of your mixture. This little addition is key to adding that creamy richness while baking.

Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 1 hour, stirring every 20 minutes to ensure even heating and flavor melding. The foil helps trap moisture, allowing the flavors to build beautifully.