Preheat your oven to 350F/180C and take the pastry out of the fridge to soften a bit. Peel onion, carrot and potato. Dice all the vegetables into similar size pieces. Put a frying pan on medium heat with olive oil, chop up the rosemary and mince the garlic and add them to the pan.
Gently saute everything stirring occasionally until your vegetables are tender and onions are translucent. It will take about 5-8 minutes. Then transfer your mixture into a bowl and cool completely.
Once it’s cooled enough to touch add the ground beef and peas. Thoroughly mix everything together adding salt and pepper and a half of your beaten egg, reserve the rest for the pastry.
Dust a clean work surface with a handful of flour and roll out the pastry to a 1/4 inch thickness or use an already rolled one.
Shape the beef mixture into a log and place it on the rolled out pastry sheet, brush the edges with the reserved egg. Roll it up tucking the ends inside like a giant burrito, brush with the egg all over and place it on a baking sheet seam down. Bake Beef Wellington in the preheated oven for 45 min to an hour depending on your oven until golden.