Allow refrigerated dough to rest at least one hour to get to room temperature.
Preheat oven to 400 degrees F. Line a baking sheet with parchment paper and spray with oil.
In a skillet over medium heat, add oil. Once hot, add onion and salt and pepper and saute, approximately 2-3 minutes. Add peppers and baby kale and cook another 2-3 minutes. Add garlic and cook till fragrant, ~30 seconds. Remove from heat and set aside to cool.
Roll out dough on a floured surface to roughly a 10 x 16 inch rectangle.
Add vegetable mixture to the dough, being sure to leave an edge of at least 1.5 inches. Top the vegetables with the cheese.
Brush the beaten egg along the edges of the dough. Fold one long side of the dough over to cover the filling, then the short edges. Be sure to pinch around the edges to keep the filling inside.
Place stromboli seam side down on the prepared pan. Cut a few small slats in the top and brush with the egg wash.
Place in the oven and bake for 25-35 minutes, until golden brown and the dough is fully cooked through.
Allow to cool at least 5-8 minutes before slicing.
Serve with marinara sauce on the side.