Step 1

out of the freezer to defrost if frozen. Preheat the oven to 425 degrees.

Step 2

, papery dirty outer layers from the spring onions. Starting with the green ends, cut the spring onions into ½-inch green tubes and then ½-inch discs with the bulb and set aside.

Step 3

to the middle of a 12-inch ovenproof pan and pour ¼ cup water over the top. Turn the heat on to medium. Cook, constantly stirring until the sugar dissolves, about 2 minutes. Then turn the heat up to high and let sit, cooking, undisturbed until the sugar starts to turn into a pale golden caramel, about 3-5 minutes – If it just turns golden on the edge of the pan, swirl that into the rest of the melted sugar with a wooden spoon and turn off the heat.

Step 4

to the caramel. Turn the heat back on to medium stir vigorously until (if using a steel pan, you can use a metal whisk) all the ingredients are combined, and the mustard is thoroughly mixed in with no visible droplets left. The caramel should be bubbling throughout this mixing. Turn off the heat once done.

Step 5

Add the onion bulb discs to the pan first; arrange them flat in the caramel all over the pan and then fill in the spaces with caramel with the green tube pieces. If any onion pieces touch the sides of the pan, tuck them in so there’s room to create a lip of crust.

Step 6

. If the dough is a circle, great—lightly flour a workspace and roll it out to 11-inches in diameter (this will fit the inside of a 12-inch pan, cover the onions, and allow for a lip of crust). If working with a pastry sheet, roll it out to 11-inches across and cut an 11-inch circle.

Step 7

over the top of the onions in the pan. The onions should be fully covered, leaving a little extra dough around the edge. Use that little extra dough and fold it onto itself to create a thicker edge. With a knife, poke holes in the pastry spaced about 1-inch apart.

Step 8

until the pastry is puffed and golden with bubbling caramel coming up the sides, about 20-25 minutes. Let cool for 1-2 minutes and quickly invert over a parchment-lined workspace or cutting board.

Step 9

, cut into slices and grate Parmesan over the top.