9-12large heirloom tomatoes(about 5 pounds or 2268 grams), sliced into ½” rounds
1large red onion(about 10 ounces or 285 grams), cut into ¼” slices
3cloves(about 6 grams) garlic, roughly chopped
3cups(45 grams) basil, roughly chopped
6cups(75 grams) parsley, roughly chopped
½cup(125 grams) Dijon mustard
½cup(115 grams) mayonnaise
8ouncessharp melting cheesesuch as Lively Run Dairy’s Lake Effect Cheddar, shredded
Adjust oven racks so that one rack is at the very bottom of the oven and one rack is in the middle of the oven. Place a layer of foil over the entirety of the bottom rack. This is to catch any drips! Heat oven to 375 degrees Fahrenheit (190 degrees Celsius).
On a lightly floured surface or between two large pieces of parchment paper, roll dough to a 22-by-18-inch rectangle. Fit into an 18-by-13-inch rimmed baking sheet, pressing into corners (pastry should hang over sides) and trim dough so that it hangs over the sides of the tray by about 1”. Use excess dough trimmings to fill in any gaps in the crust. Refrigerate crust while assembling filling.
To make a lattice top (optional), reroll remaining dough scraps and cut into 6 even strips. Place strips on a parchment paper lined tray and in the fridge until ready to assemble.
Assemble filling: place sliced tomatoes on a paper towel-lined tray and sprinkle evenly with 1 tablespoon kosher salt. This will help to draw moisture out of them so that the pie doesn’t get soggy as it bakes. Set tray aside at room temperature while you prepare the rest of the filling.
Combine garlic, parsley, basil, and 1 tablespoon kosher salt in a food processor. Pulse until finely chopped.
Add mustard and mayonnaise and pulse until a spreadable paste forms. You may need to scrape down the sides of the food processor a few times. Taste and adjust seasonings as necessary-- it should taste on the saltier side at this point.
Blot tomatoes with paper towels to remove excess liquid.
Remove chilled crust from the fridge. Using a small spoon or offset spatula, spread mustard herb mixture evenly on the bottom of the crust. Evenly distribute shredded cheese on top. Top with tomatoes, overlapping. Cut smaller pieces of tomatoes to fill gaps. The tomatoes shrink while roasting, so keep them tightly packed. Sprinkle onion slices over the tomatoes.
If using, lay lattice pieces over tomatoes and onions. Roll up overhanging edges of the bottom crust around the sides of the sheet tray and crimp tightly by forming a v shape with your thumb and finger on one side of the crust and pushing your other index finger into that v from the other side. Whisk 1 egg with 1 tablespoon water until completely homogenous. Gently brush egg wash over the crimped edges and across the lattice.
Bake pie: Place assembled slab pie on the middle rack of the oven and bake for 40 minutes. After the 40-minute mark, rotate pie and bake for 20-30 minutes more, or until the crust is deeply browned and the tomatoes are softened and even a bit blackened in spots.
Allow the pie to cool on a wire rack for an hour before slicing and serving. To slice, cut pie in half lengthwise and then cut each half in half again. Rotate pie and cut in half widthwise and cut each of those halves in half again, for a total of 16 slices. Serve warm or at room temperature.