How to Make It
Heat oven to 400° F. Heat the oil in a large skillet over medium-high heat. Add the sausage and cook, breaking it up with a spoon, until cooked partway through, 3 to 4 minutes. Transfer to a plate using a slotted spoon; reserve the skillet.
Add the zucchini, 2 tablespoons water, ½ teaspoon salt, and ¼ teaspoon pepper to the drippings in the skillet. Cook, stirring occasionally, until just beginning to soften, 4 to 5 minutes.
Unfold the pastry on a baking sheet and prick all over with a fork, leaving a ½-inch border around the dough. Spread the mustard on the pastry (avoiding the border); top with the zucchini and sausage. Bake until the pastry is golden brown, 20 to 25 minutes.
Cut the tart into 8 pieces. Serve with salad.