Heat the oven to 400F. Place a baking stone or half sheet pan on the center rack to heat. (The half sheet pan is slightly larger than the quarter sheet pan used for the slab pie. Preheating a baking stone or half sheet pan and then placing the slab pie pan on it when ready to bake helps brown and crisp up the bottom of the crust. You can skip this step if you don't have either.)

Cut a piece of parchment paper to fit the slab pie pan (9 x 13-inch baking sheet). Dust lightly with flour and use it as a guide to roll out the puff pastry to size. With a sharp knife, score the surface of the pastry nearly all the way through the dough in a tic-tac-toe pattern, to mark off either 8-12 portions. Slide the parchment with the pastry into the slab pie pan and refrigerate.

In the bowl of a stand mixer fitted with a paddle, beat the cream cheese on medium speed until smooth. Scrape down the sides of the bowl and add the sugar, salt, vanilla, and 1/2 teaspoon of lemon juice and mix on low until completely combined. Taste the filling, and add a little more lemon juice if the flavor is dull. (The filling shouldn’t taste like lemon, but a little bit of lemon juice will add a bright note to the filling. When tasting, look for a bit of a zing in your mouth that doesn’t scream lemon.)

Use a small scoop or two spoons to plop cheese filling on top of each portion, no more than 1 - 1 1/2 tablespoons each. Less is more: Too much cheese will overwhelm the pastry and tamp down the layers, so hold back.

Use the back of a spoon to make a well in each dot of the cheese and spoon in a scant 1/2 teaspoon of jam. Place the slab pie (in its pan) on top of the heated baking stone or the half sheet pan. Bake until the cheese is beginning to brown here and there and the pastry is toasty brown, 21-23 minutes.

Cool the cheese danish slab pie for no more than a moment. Dust with the powdered sugar. Ideally, slice and serve this pie barely warm, using a sharp scissors or pizza wheel to cut the portions along the score lines.