How to Make It
Combine all-purpose flour, butter, buckwheat flour, and kosher salt in a food processor, and pulse until butter pieces are no larger than pea size, about 5 times. Add 1/3 cup ice water, and process just until dough comes together in a shaggy mass, about 6 seconds.
Line a large rimmed baking sheet with parchment paper; set aside. Transfer dough to a lightly floured work surface; gather dough, and pat into a smooth ball. Using a floured rolling pin, roll out dough to a 15-inch circle, rotating dough occasionally to make sure it isn’t sticking and dusting with additional flour as needed. Loosely roll dough circle onto rolling pin; unroll onto prepared baking sheet. Cover with plastic wrap, and refrigerate until cold and firm, about 20 minutes.
Meanwhile, preheat oven to 400°F. Heat oil in a large skillet over medium-high. Add sausage and crushed fennel seeds; cook, breaking up sausage with a wooden spoon, until meat is browned, about 5 minutes. Remove skillet from heat; add squash, a few grinds of black pepper, and 1/2 teaspoon kosher salt; stir until well combined.
Stir together ricotta, oregano, honey, and remaining 1/4 teaspoon kosher salt in a medium bowl.
Remove chilled dough from refrigerator. Spread ricotta mixture over dough, leaving a 2-inch border. Scatter squash-sausage mixture in an even layer over ricotta mixture. Fold edges of dough over filling, overlapping every 4 inches or so to create an evenly pleated crust. Brush crust lightly with some of the beaten egg, and sprinkle generously with flaky sea salt.
Bake in preheated oven until nicely browned and crisp, 40 to 45 minutes. Remove from oven, and immediately drizzle generously with honey. Scatter oregano leaves over filling, and, if desired, sprinkle with red pepper. Cut crostata into wedges.