Preheat the oven to 220°C (428°F) with top and bottom heat.
Roll out the defrosted puff pastry into a rectangle approximately 30 x 40 cm.
Finely chop the onion and finely dice the red and green bell peppers. Heat 2 tablespoons of oil over medium heat, add the onion and peppers, and sauté for 10 minutes until softened.
In a bowl, mix the 400 g minced beef with 1 egg, 2 teaspoons salt, black pepper, and 2 tablespoons tomato ketchup. Mix in the cooked vegetables. Spread the mixture evenly over the puff pastry. Scatter the diced Emmentaler cheese over the filling. Roll up the pastry tightly.
Brush the strudel with egg yolk and press the edges firmly to seal. Place on a baking sheet lined with baking paper.
Slide into the preheated oven and bake at 220°C (428°F) for 10 minutes, then lower the heat to 175°C (347°F) and bake for another 25 minutes.