Hollow out the vegetables, as follows:

Bell pepper: cut about 1/2 inch top off. Remove the seeds from the pepper, and rinse out the pepper if some seeds remain inside.

Eggplant: cut the stem part off the end, then place it lengthwise on the cutting board. Cut a thin slice off of one side, lengthwise, to create a stable base. Cut about 1/4-1/2 inch, lengthwise, off the opposite side (the "lid"). Leaving about a 1/4 inch perimeter, hollow out the inside of the eggplant. Refer to the pictures if you're wondering how to do this.

Squash: Cut off the stem side. Leaving about 1/8-1/4 inch perimeter, hollow out the inside. Again, refer to the pictures above if you're having trouble visualizing this. I cut the squash crosswise, in comparison to the eggplant, which I cut lengthwise. Slice a thin piece off the base so the squash can stand upright in the pot. If you have a long squash, cut it in half, and hollow out each half separately.

Set all vegetables aside.

Blanch the cabbage leaves: bring a pot of water large enough to fit the largest cabbage leaf to boil over medium/high heat. Boil the cabbage leaves in batches of 3 or so at a time for 1-2 minutes. Drain, then set aside. If there is a thick rib at the bottom of the cabbage leaf, cut it out.

In a bowl, combine ground beef, 1/3 cup tomato puree, rice, onion, salt, and pepper. Mix with hands to combine.

Fill each of the vegetables with the meat mixture, and place in a pot that is large enough and deep enough to hold them (I used a Le Creuset 5 qt pot).  For the cabbage leaves, place about 1-2 tablespoons of meat mixture at one end of the leaf (I use use the cut end). Fold the end down over the meat, then fold in the sides. Continue to roll the leaf until you have rolled it up completely, almost like a burrito. Place it seam-side down in the pot.

If you have leftover meat, make a few meatballs and place them in the pot. Put the "lids" on the veggies (I did this for the pepper and eggplant). Cover with tomato puree; it should come at least halfway up the vegetables. Put the lid on the pot and put on medium heat, turning down if the pot begins to bubble over. Simmer for 45 minutes, or until meat is cooked through.

Serve each vegetable in a bowl with some tomato sauce. Top with a dollop of plain yogurt, and some sumac or Aleppo pepper, if you'd like.