Put the bulghur in a large bowl and stir in 2 cups boiling water. Cover tightly and let stand until the bulghur is tender and most of the water has been absorbed, 20 to 30 minutes.

Drain in a strainer, then pour the mixture onto a large kitchen towel and squeeze out any excess liquid. Return the bulghur to the bowl.

Cut off about 1/2 inch the top of each tomato. With your finger, poke the seed pockets and remove the seeds and juice. With a small sharp knife, cut out the inner pulp. Dice the pulp and discard the seeds.

Whisk the lemon juice, olive oil, salt, coriander, and black pepper in a large bowl and stir to blend. Add the chopped tomato pulp, cucumbers, green onions, parsley, walnuts, mint, and bulghur and mix well.

Stuff the tomato shells with the bulghur mixture. Serve at room temperature or chilled.