The avocado is optional, but adds a nice creaminess to the filling. If the arugula is too peppery for you, substitute up to ¼ cup of chopped parsley.
Slice off the stem end of the tomatoes and carefully remove the pulp using a sharp knife and a spoon. Discard the pulp and set the tomatoes aside.
In a medium bowl, combine the arugula, feta and avocado, if using. In a small bowl, whisk together the olive oil, lemon juice, salt and pepper. Pour the olive oil mixture over the arugula mixture and toss gently, breaking up any large bits of feta. Taste the filling, then add more salt, if needed. Feta varies in saltiness; you may not need more.
Place the tomatoes on a serving plate and stuff with the arugula mixture. Garnish with chopped pistachios and serve.
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