Preheat oven to 400*F. Line a baking sheet with parchment paper and lay out sweet potato and red onions. Drizzle with 2 tbsp EVOO and ¼ tsp fleur de sel and cracked pepper, and roast for 10 minutes. Remove and let cool.
On a lightly floured sheet of parchment paper, roll out pastry to a 10-12" circle, turning over halfway through rolling to prevent sticking. Move parchment paper and rolled out pastry to a baking sheet.
Spread ricotta cheese evenly in centre of pastry, leaving a 2-3" border of pastry, then top with onion confit, spreading evenly. Spread kale on top of ricotta, then arrange sweet potatoes, apple slices and onion over kale, keeping the 2-3" border intact. Finishing by sprinkling mozzarella cheese over filling and begin gently folding up perimeter of pastry, folding edges over itself. Sprinkle filling with remaining fleur de sel and pepper and brush pastry border with egg/water mixture to help seal edges and give pastry a glaze. Drizzle galette with remaining olive oil.
Bake galette for 25-30 minutes, until crust is golden brown. Let cool on a rack before moving with two spatulas to a cutting board. Cut into wedges and serve warm.