In a large 5 quart pot filled 2/3’s full of water, bring to a boil with a dash of sea salt. When the water is boiling, carefully place the cabbage head into the water for 2 minutes. With tongs, pull the head of cabbage out of the water and rest on a large cutting board.
Carefully remove the softened outer leaves, keeping them whole; set the leaves aside. Return cabbage head back to the pot and repeat until you have 12-16 cabbage leaves.
Finely chop the remaining cabbage, measuring out 2 cups and set aside.
If using a food processor, place the onions and process until they are finely chopped. Otherwise, finely chop by hand.
Heat a large skillet over medium-low heat and add the olive oil. Add the onion and sauté 5 minutes, stirring occasionally. While the onion cooks, add the carrots, mushrooms and garlic to the food processor and process until they are finely chopped. Otherwise, finely chop by hand.
Increase heat to medium-high. Add the 2 cups of chopped cabbage, carrots, mushrooms and garlic; sauté for 4 minutes. Remove from heat; set vegetables aside to cool slightly.
To a large bowl, add the brown rice, uncooked ground beef, thyme, paprika, salt, pepper, and egg. Stir gently. When the sauteed vegetables have cooled, add them to the large bowl and combine gently.
Spread 1 cup of the marinara sauce on the bottom of a 13 x 9-inch baking dish.
To the cabbage leaf, Place approximately 1/4 cup for Napa or 1/2 cup for Savoy of the cabbage-meat mixture in the center of each leaf. Starting at the core end, roll into a tight log, tucking in the sides of the cabbage leaf as you go. Place cabbage roll in the baking dish, seam side down. Repeat with the remaining leaves and filling.
Top with remaining marinara sauce.
Cover the baking dish with foil; bake at 350 degrees for 45-minutes. Uncover and serve warm.