Preheat the oven to 400°F (200°C).
Season the turkey breast evenly with salt, black pepper, dried sage, and garlic powder.
Heat olive oil in a skillet over medium-high heat. Sear the turkey breast for 2-3 minutes on each side until golden brown. Remove from heat and let cool slightly.
On a lightly floured surface, roll out the puff pastry sheet to a 10×10 inch square.
Spread the prepared sage and onion stuffing evenly in the center of the puff pastry, leaving a 1-inch border around the edges.
Place the seared turkey breast on top of the stuffing.
Spread the cranberry sauce evenly over the turkey breast.
Fold the puff pastry edges over the turkey and stuffing, sealing all edges by pinching or pressing with a fork to form a neat parcel.
Place the Wellington seam side down on a baking sheet lined with parchment paper.
Brush the entire pastry with the beaten egg to give it a golden finish.
Using a sharp knife, lightly score the top of the pastry with a decorative pattern, being careful not to cut through.
Bake in the preheated oven for 40-45 minutes, or until the pastry is golden brown and the internal temperature of the turkey reaches 165°F (74°C).
Remove from oven and let rest for 10 minutes before slicing and serving.