Preheat the oven to 350 degrees F. Place the thawed puff pastry in a 8- or 9-inch tart pan and crimp the edges. Remove the excess dough and piece it along the rim if needed to make the rim even in some places. Refrigerate the crust until ready to fill.
Place the butter and bacon in a large skillet over medium-high heat. Sauté until the bacon is crispy then toss in the onions, followed by the potatoes and dill. Season with 1/2 teaspoon salt and 1/4 teaspoon ground pepper. Gently stir 3-5 minutes,to mix the onions and potatoes and coat in bacon grease. (The potatoes don't need to be cooked through.) Then spoon the mixture into the crust and drizzle with heavy cream.
Bake on the low rack in the oven for 35-45 minutes, until the potatoes are fork-tender and the crust is golden. Rest for 10 minutes, then sprinkle with chopped chive and cut.