Place the sweet potato slices in a bowl, drizzle with olive oil, season with coarse salt, and toss to coat evenly. Set aside.

Heat 1 to 2 tablespoons of olive oil in a skillet over medium heat. Add the onions and sauté until golden, about 10 to 12 minutes. Add the spinach and cook until wilted. Pour in the cream, season with salt and pepper, and bring to a gentle boil. Remove from the heat and stir in the cream cheese until the mixture is smooth and well combined.

Preheat the convection oven to 355°F (180°C).

Unroll the puff pastry onto a sheet of parchment paper. Trim it into a rectangle, reserving the excess dough. Use the excess dough to form a border around the edges of the rectangle, pressing gently to secure.

Spread the spinach mixture evenly over the center of the pastry. Arrange the sweet potato slices on top in a slightly overlapping layer. Scatter the feta cubes evenly over the sweet potatoes.

Transfer the assembled pastry on the parchment to a baking sheet. Bake for 30 minutes, or until the pastry is golden and puffed. Let cool slightly before garnishing with fresh arugula leaves. Serve warm or at room temperature.