Combine the cracked wheat, onion, tomatoes, and olive oil in a large bowl. Season with salt, pepper, and the cumin. Cover and let rest for 2 hours, then mix in the dill and mint.
While the wheat mixture is resting, drain the grape leaves and rinse well in a colander. Bring a large pot of water to a rolling boil and blanch the grape leaves, in batches if necessary, for 4 to 5 minutes. Drain and immediately rinse under cold running water. Trim the tough stems off the leaves and set aside the ones that are either very small or torn. Place a heaping teaspoon of filling in the bottom center of each leaf on the table. Fold the bottom over the filling, then fold the sides over it and roll up, tucking the sides in as you go. Place the grape leaves seam side down in the pot, snugly next to each other, in layers. Pour the lemon juice over them, then pour in enough water to barely cover the surface of the dolmathes. Cut a piece of parchment to the circumference of the pot and fit it over the leaves. Cover with a plate to keep the grape leaves in place and place the lid on the pot. Simmer until the leaves and cracked wheat are tender and the pot juices absorbed, about 50 minutes. Remove from the heat and serve.
How to Grate Tomatoes: This is one of the great rustic techniques of the Greek kitchen. Have ready a hand grater, preferably one with coarse teeth. Make sure the tomatoes are washed and dried. Hold the grater over a plate or shallow bowl. Hold each tomato from the stem end and grate it along the coarse openings of the grater until all that remains is the tomato's skin, flat and wide open. What you are left with are the pulp, the seeds, and the juice. As for the seeds, most traditional cooks aren't bothered by them.
Recipe courtesy of The Glorious Foods of Greece (Morrow, 2001) by Diane Kochilas