For the dough: In a food processor, pulse to combine the flour and salt. Add the butter and pulse 3 to 4 times, then drizzle in the ice water and pulse until the dough holds together easily when squeezed with your fingers. Add more ice water, 1 teaspoon at a time, if needed. Divide the dough in half, pat each portion into a round disk, wrap with plastic wrap and chill in the refrigerator for at least 30 minutes.
For the salad: In a small jar with a tight-fitting lid, combine the oil, vinegar, mustard, parsley, tarragon, sugar, shallots and some salt and pepper. Shake to mix the dressing and chill until ready to use. Place the greens in a large bowl, drizzle about 1/4 cup dressing over and toss to combine.
For the tart: Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper.
Toss the tomatoes with the oil, onions and some salt and pepper, then set aside.
On a lightly floured surface, roll one disk of the dough into a 12-inch round, about 1/8-inch thick. Transfer to the prepared baking sheet, then fold and crimp the edges.
Spoon the sour cream and mustard evenly on the dough. Top with the tomato mixture and the goat cheese. Make an egg wash by beating the egg with 1 teaspoon water. Brush the egg wash around the tart edges.
Transfer the tart to the oven and bake until the crust is golden and the tomatoes soften, about 45 minutes. Cut into squares to serve with the salad.
This recipe makes enough dough for two tarts.