In a large skillet, heat the 1 ½ tablespoons oil over medium-high heat. Add the sausage and cook, breaking up with a wooden spoon, until browned. Transfer to a plate.
Reduce the heat to low. Add the remaining oil, fennel, and garlic; season with salt and pepper to taste. Cover and cook, stirring occasionally, until the vegetables soften, about 5 minutes. Add spinach and stir until wilted. Mix in the cooked sausage.
Preheat oven to 425 degrees Fahrenheit.
In a small bowl, beat the egg with 1 teaspoon water. On a lightly floured surface, roll each puff pastry sheet into a 10x12-inch rectangle. Transfer one sheet to a parchment-lined baking sheet. Top with sausage mixture, leaving a 1-inch border. With a tablespoon, dot with the ricotta, then drizzle with the marinara sauce and sprinkle with half the basil.
Brush the border of the pastry with the egg wash and line up with the remaining sheet of puff pastry, pressing the edges to seal.
Brush the top with egg, and then use a fork to crimp the edges. Cut 4 slits in the top to allow steam to escape. Bake until the pastry is puffed and golden, about 20-30 minutes. Sprinkle with the remaining basil and serve with a dollop of sour cream, if desired.
You can use mild sausage if you're not a fan of spicy food.
If you don't want to use Italian sausage at all, then you can substitute with regular ground beef or pork. Don't eat pork? Use spicy kosher/halal chicken or turkey sausage if you'd prefer. In that case, add some extra herbs - like perhaps Italian seasoning, some extra garlic, and maybe a pinch of chili flakes. The pie will still be delicious either way!
It's essential to make sure you slit the top of the pastry so that it remains light, buttery and crisp as otherwise, the trapped steam on the inside will make the pastry soggy.
Place the slab pie on a sheet pan larger than the pie, so you have enough room to work on crimping the edges. I have rolled out the first pastry on a piece of parchment paper and for easy transferring. Alternatively, assemble the whole pie on a piece of parchment paper and then gently transfer to the baking pan.
Roll out the top pastry sheet slightly larger than the first (by ca. ½-inch) so that it can cover the filling and the bottom sheet.
To slice (julienne) basil, wash the leaves and stack them together like sheets of paper. Gently roll up the stack into a loose cigar shape. Using a sharp knife, slice across the roll to make very thin "chiffonade" (hold off prepping basil until the last minute because once cut or chopped, basil will discolor very quickly).
For the best results, read additional tips in the post above.
Please keep in mind that nutritional information is a rough estimate and can vary significantly based on products used.