Add butter into pan over medium-high heat. Fry onions for 2 minutes then add garlic and fry for another minute.
Add spinach and mix well. Add salt and black pepper.
Add breadcrumbs, cream cheese and parmesan cheese. Mix until well combined. Remove from heat and set aside to cool.
Pre-heat oven to 375 degrees F. Beat egg in a bowl.
In a mixing bowl, mix drained tuna and mayo until texture is uniform,
Lay out puff pastry and brush with egg wash on one long edge.
Dish out half of the spinach mixture and place in the middle of the puff pastry leaving some space at the top and bottom edges. Place half of the tuna on top of the spinach.
Wrap the edge that does not have egg wash over the tuna. Wrap the egg-washed edge over the first wrap. Use a fork to seal the top and bottom edges of the pastry.
Place the wellington in a baking dish lined with aluminum foil. Repeat for the second wellington.
With a knife, make small slits on top of the wellington. Apply egg wash over the wellingtons.
Bake in pre-heated oven for 30 minutes.
