Bring a large pot of water to a rolling boil. Carefully drop the cabbage into the pot and let it boil for about 5 minutes.
Put the cabbage in a colander set over a large bowl to drain and cool slightly. Remove the outermost layers of leaves, until you reach leaves that aren't pliable.
Repeat the boiling and draining process until you have 8 cabbage leaves. Pat the leaves dry with paper towels and set aside.
To a medium bowl, add the lentils, rice, onion, mushrooms, pine nuts, raisins, garlic, dill, olive oil, and salt. Stir gently until well-combined.
Add the marinara sauce and ¼ cup water to a 3-quart or larger Crock Pot. Stir to combine.
Set a cabbage leaf on a working surface, outer side down, and scoop about ½ cup of the mushroom-lentil mixture into the middle. Fold the stem end up about ⅓ of the way, then fold in the sides. Continue rolling semi-tightly until completely rolled up. Lay roll seam-side down on top of the marinara sauce.
Repeat with remaining cabbage leaves, piling each into the slow cooker in one or two layers, right on top of one another.
Cook on HIGH for 4–5 hours or on LOW for 8–10 hours.
To serve, scoop out a little marinara and spread it on a plate. Carefully remove cabbage rolls one at a time and transfer to the plate. Ladle an additional spoonful or two of the marinara sauce from the slow cooker over the top of each roll and top with a few sprigs of fresh dill, if desired.
* To toast the pine nuts, place a medium sauté pan over medium heat and add the pine nuts. Cook, stirring frequently, until toasted and just beginning to get fragrant—about 4–5 minutes. Watch the nuts closely and stir constantly toward the end of the toasting process, as they can burn easily!