Preheat the oven to 400 degrees F. Line a baking sheet with parchment.
Unfold the puff pastry, and cut it into 4 equal squares (you don't have to roll the dough out). Arrange the squares, evenly spaced, on the prepared baking sheet. Measure about 1/4 inch in from one side of each square, and score the dough all down that side almost all the way through with a sharp knife. Repeat with the remaining 3 sides, making sure the cuts meet in the corners (you're making a square within a square). Poke each inner square several times with a fork.
Bake the dough until puffed and lightly golden, 15 to 20 minutes. Let cool for a few minutes, then press on the center of each puff with the bottom of a measuring cup to make a well. Let cool completely. (These shells can be made up to 3 days in advance and stored in an airtight container at room temperature.)
Crack 1 egg directly into the well of each pastry shell. Dividing evenly, arrange the tomato halves cut-side up on the egg whites. Sprinkle the egg and tomato with salt and pepper. Bake until the egg whites are just set but still slightly jiggly and the yolk is still runny, 15 to 20 minutes.
Meanwhile, whisk together the harissa, 1 tablespoon water, 1 teaspoon of the oil and a pinch of salt in a small bowl. Toss the herbs, feta, scallions, lemon juice, the remaining 1 teaspoon oil and a pinch of salt in another small bowl.
Drizzle each baked tart with some of the harissa sauce, and top with a little mound of the herb salad.
Harissa-a North African sauce made from chile peppers, cumin and other seasonings-varies greatly by brand and can be super spicy. Be sure to taste yours before using and adjust the quantity accordingly. Never leave a child unattended in the kitchen. Limit the child to tasks that are safe and age-appropriate.
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